Hi,
do you need to adjust the level of a really large ring-sized spell by a size-modifier?
Eg. you want to cast a CrHe-Spell around an immense crop field (with the help of some CrIm or CrTe to draw an exact circle) - do you have to calculate the big size for an additional size-modifier? Or is it still Range: Touch (+1 lvl) Duration: Ring (+2 lvl) Target: Circle (+0 lvl)?
Thanks
Technically, the ring can be any size at that one Target without extra magnitudes. But be careful! This gets dangerous. Even if you use magic, you have to physically trace out the ring. 10 paces/round (5 ft/s) is the limit to how quickly that can normally be done. Each round of extension requires a Concentration roll of 6+. So let's say you want a ring 300 or so paces across. That's about 94 rounds of extension. Let's say you only have 1 botch die on your Concentration roll. Your chances of not botching a Concentration roll are 0.99[sup]94[/sup]=0.39, meaning you have about a 61% chance of botching at least one Concentration roll. I would expect failing the Concentration roll would lose you the spell, while botching it might well botch the spell. Add to this that if anyone breaks the ring while you're casting the spell you automatically botch the spell. I would start thinking about subdividing the field or going with a D:Year, T:Boundary spell...
Chris
I would say "Yes, but..." Keep in mind that, when you're casting a T: Circle spell, it's bound by many of the same rules as casting a D: Ring.
The two that speak against having a Circle-target spell around a field are (1) the spell ends whenever the circle is broken, even if it's before the duration of the spell ends, and (2) You must make a Concentration roll every round you're drawing the circle.
For 1, I'm assuming that the magus is simply drawing a circular furrow around the field, and that won't last past the next good rain, or someone dragging something across the furrow. If you have the foresight to, say, have a more permanent ring in place (a circle of logs, say, or metal chains laid together in a ring or something), you should be fine there.
For 2, if it were my saga, and a player wanted to try that, I'd say "Fine. How big is the field? 150 yards across? That's 1,413 feet circumference, that'll take you 47 rounds to draw. Okay, give me 47 Concentration rolls." Granted, the target is only a 6+, but you've got a 47% chance of botching one of those rolls, and given average (+1) Int and a half-way decent Concentration score (3), you only have about a 2/3 of 1% chance of not failing at least one of those 47 Concentration checks.
Better off doing it either Boundary (and, considering that the Base Boundary is only a 300 foot circle, with probably Size modifiers for that), or dividing the field up into smaller plots and casting the spell multiple times.
As has been mentioned, one can make a really, really large Ring without paying any extra magnitudes, with the two caveats that a) the spell ends if the Ring is broken and b) the caster needs to make a large number of concentration rolls for large Rings.
Accidental Ring breaking is usually not an issue if one takes the appropriate precautions by making the Ring sufficiently "durable" (e.g. a low circular stone wall surrounding the field one is blessing). However, actively malicious disruptions are far, far harder to prevent.
Concentration rolls are less of an issue than other posters suggest, unless the Ring is cast in a difficult/dangerous situation: nothing says that the Concentration roll is necessarily made on a stress die, so if the situation is one of relative calm etc. you can safely assume that any magus with an Int+Concentration total of 5 or more (including any relevant Concentration specialty such as the very common -- and canonical -- "spell concentration") can trace really large rings.
Xavi wonders how many concentration rolls would be needed to surround most of Iberia with a Ring...
About half a million -- over a month of non-stop walking and spellcasting. That's really too large, both for the need to keep awake (though you could probably use ReVi to sustain your spell while you sleep) and because anything breaking your circle while you trace it means a botch. However, a really gigantic field, one mile in diameter, could be "encircled" in about 500 rounds -- less than an hour; so it's probably quite feasible in a calm situation with an Int+Conc total of 5+ (including specialties).
A circle with a diameter of a mile has a circumference of 3.14 miles.
To make that walkable in an hour, I'd make them put a road around the field, which would look a bit odd and require a fair bit of effort in advance. Further, I really think we're discounting how hard it is to walk and concentrate on something else at the same time. My walking speed on my treadmill, when I'm concentrating on walking is ~4mph, when I'm playing a game, it drops to around 2mph. If it were uneven terrain, not prepared, 1 mph or slower is not unreasonable...
There's a reason why The Bountiful Harvest is a boundary spell... 
Trying to circumvent that should be an exercise in near futility, or at least have costs that approach, if not exceed the spell it is trying to replicate (which is what I'm assuming based on the OP mentioning it is CrHe).
I don't think it's that easy to keep walking along while tracing something out and concentrating on a spell. I'll use a real-world example for why. A nearby town recently outlawed texting while walking. Yes, texting while walking is a petty crime for which you can be fined there. The reason that was done is because there were so many accidents occurring as a result of texting while walking, including a couple deaths. To me that says stress roll with a single botch die.
Chris
By the RAW, it's up to 10 paces (i.e. yards)/round. 3.14 miles is slighly less than 5530 yards, or 553 rounds. Each round is 6 seconds, for a grand total of 3318 seconds, or 55 minutes and 18 seconds. Granted, these numbers are approximate, but they are in the ballpark I mentioned. Sure, if it's dark, the terrain is uneven, the magus is drunk etc. it's going to be harder. But it's fairly easy, particularly with a little helping magic.
Risky? Well, somewhat. Definitely nothing you want to do in a rush, or if you suspect someone/something might cause trouble. Anything that disrupts the 3+mile circumference breaks the spell. Anything that does so during that 1 hour walk makes the spell botch. But I'd say under certain circumstances it would be very reasonable to attempt it.
The point is that you are not tracing something out while concentrating on a spell. You are concentrating on the spell for which you are tracing the ring. It's a little like saying: it's hard to drive while you are staring out of the car's window -- but driving while staring out of the car's front window at the road is not hard, it's how you are supposed to drive. So I don't think this should be unconditionally a stress roll. The RAW state explicitly that other types of rolls are always stress; in the absence of such an explicit statement there are circumstances which make the roll simple.
Now, if the road is crowded with people, vehicles etc. (as the roads in your neighbourhood certainly are) then, I agree, you may have botch dice. Just as if it's dark, or there's a really strong wind buffeting the caster, or the terrain is slippery etc. But a magus generally can, without superhuman effort, ensure that there are no such aggravating circumstances.
Your analogy to the car and driving fails because driving is the activity which requires constant observation. Introduce texting, checking email or even talking on one's cell phone and the results become catastrophic rather easily. Casting a spell while walking quickly to trace the circumference of the circle is closer to distracted driving than it is to regular driving. The risk of botch is real, especially if it is difficult to perform the activity again...
I'd stand by my statement of 1 mph to trace the circle while casting the spell, especially a ridiculously large spell over uneven/unprepared terrain. Keep in mind that the magus is concentrating on maintaining control of the magic and looking at/tracing the circle. He is not paying attention to the terrain he has to walk over.
If the circle is small enough, I would totally agree. But you seem to have left out the whole walking (or flying, or whatever) part. And ArM5 specifically considers it harder to concentrate on casting a ring spell while traveling around the ring than it does casting a typical, non-concentration spell while walking. And this is while walking at a 17.6-min mile, so it's not like we're considering jogging here is making for a higher difficulty than walking.
Chris
I'll say again, for the last time: casting a spell while walking is one thing -- casting a spell while walking to cast that spell is another. That's exactly what my driving analogy was meant to cover.
As I said, if the terrain is treacherous I totally agree that the magus will be slowed down, and possibly risk a botch. The same if it's dark, or if there are lots of children or animals frolicking around etc. But inideal* conditions, which are not that hard to obtain with a little preparation and magic, I disagree. Do I risk accidents by texting while walking around in a city? Absolutely, I risk bumping into other people/objects. Do I risk accidents by texting while walking down a straight empty corridor? I may be optimistic, but I don't think so.
I'm not sure I'd entirely agree.
Casting while walking is a stress roll of Stamina+Concentration, with an ease factor 3.
Maintaining concentration on a Ring spell you are casting is a "plain" roll (which I take to mean either simple or stress, depending on circumstances) of Intelligence+Concentration, with an ease factor 6.
Walking isn't specifically part of casting a T:Ring spell. You can cast a T:Ring spell sitting still, just tracing out a ring with your finger.
Chris
Table on page 7: 6 is closer to "hard" than is 3. Ars Magica doesn't not define easy/hard based on botch dice. Botch dice reflect that if something goes wrong it has a greater (or lesser with fewer) chance to go terribly wrong. For example, balancing on two identical slack ropes would have the same ease factor. But if one is just above soft ground and the other is tied between the 10th floors of two buildings and over a street below, the second would have lots more botch dice.
Generally, if concentrating in some way on a spell is part of the spell the ease factor gets lowered from what it would be with another spell. This shows up for both D:Concentration spells and Maintaining the Demanding Spell. Yet in this case the ease factor goes up instead of down. Why is that opposite to the standard if walking while tracing the ring is inherently part of the spell?
Chris
True, but I guess it is much more common (and studied) how to cast a Ring spell while walking than casting a room spell while doing the same thing 
Since basic ability difficulties for Ars go up to 24, and that a totally untrained average person will pass a difficulty 6 in the Ars system about 40% of the time I have a difficult time imagining that difficulty as "hard", really. More on the "dirt easy" side than "hard", really
Xavi
Yet it's harder to do so with the Ring spell than it is with the Room spell - ease factor 6 vs. 3? Why in all (to my knowledge) other cases when something is specifically part of a spell is doing that thing easier than normal while casting, yet in this case the situation would be reversed? So either these are beyond and exception and actually a contradiction to the standard, or the interpretation that walking is part of the standard action of spell casting must be off. I haven't seen anything that suggests walking is a standard part of casting a Ring spell, but I have presented evidence from the rules that it is not.
The way I'm picturing it at this point based off the wording and based off the ease factors is that with a spell like the Room spell you are walking while you're casting, but with a Ring spell you're tracing your finger (or talisman, etc.) along a circular path, trying not to lose contact as you walk. That wouldn't be terribly difficult, but it would certainly be more difficult than walking and not tracing.
Notice, I didn't say "hard." I said "harder." 6 is closer to "hard" than is 3. (Next part from memory.) But 6 is still "easy." 3 is "simple," and 0 is "trivial," both being less difficult than "easy." The point is that this is harder to do than walk and cast a normal spell.
Chris
Ah, I see. Point taken. For some strange reason our mages never seem to walk and cast spells. They might be running , falling or actively dodging while casting spells, but not walking. Heh.
No problem for me to this... i mean, if it works on groups, this it should work one big beings, one of my projects. Again you must draw and use Concentration.