I have a player who's thinking about looking getting an especially small magical creature to be his familiar, something mouse sized or smaller. What kind of advice would you give for creatures like a cricket, butterfly, or mouse?
Hey, you posted this twice! Delete the other thread!
You could give the creature high Might just to be funny about it. But really, no, tiny Familiars aren't any different from bigger Familiars except in how hard they are to bind. I'd probably give it a neat power or two that conveniently makes it more sensible as a lab assistant, though. Just a thought.
First thing you need to think about is a house rule for the Familiar Bonding Level. As described on ArM5, page 104, the level is equal to Might + 25 + (5 x Size). If you go for an insect at Size -15, you have a Bonding Level of (Might - 50), which is almost certainly going to be negative. This means that any magus could do this, but what does it mean for Might cost? This is normally 1 pawn per 5 points of the level, but what to do if the level is negative? You might want to consider a minimum level of 0, or a minimum cost of 1 pawn.
Secondly, I think you might find that a really small familiar is a liability. They are too easy to kill at this size, even accidentally. I would perhaps consider having a conversation with your player about the problems of an extremely small critter, and maybe encourage him towards thinking about that mouse, or a songbird. Still not easy to play, but at least you can give it stats!
Mark
What about a butterfly with a 60 Might?
The player was suggesting the idea of tiny sizes in order to get a familiar with the highest Might at the start, using the Magical Animal Companion flaw. Judging by the math, keeping a positive level requires a size of -7 for the creature, which is Might 17.
If you want to go small, yet a bit more meaningful, why not a Bee queen/king, allowing the control of the whole hive ?
A tiny butterfly with might of 60 is just abusing the rule to bind an extremely powerful creature, without having to pay the "real price".
Another way to look at it, is why such a mighty creature would like to bind herself to servitude with such a lowly creature as a magus ?
To put that in perspective, why a dragon with the equivalent MM of 60 would accept to become a familiar (which amongst other thing means very likely dying when the magus will pass away, losing his immortality in the process) ?
At a MM of 60 size is inconsequential as very likely the creature has a power allowing her to grow or take another shape, and has many powers to do lots of magic. What would be the benefit to be bound to a mortal magus ? I would more see the creature binding a magus to servitude 
than becoming a familiar, would he see the benefit of such alliance.
Even if there is no rule mechanism, the first step to bind a familiar is to earn his trust (befriending him), and there must be a "mutual admiration". A creature with a MM of 60 must be intelligent and/or wise - I cannot see such a mighty creature behaving like a normal animal, or just have cunning. So if such a might butterly ever existed, he should be considered the equivalent of a dragon, or faerie lord.
One shouldn't use the binding formula as a basis for designing a creature to be bound. That's ignoring the character creation guidelines in RoP:Magic. A might 60 character is technically a magus level character in a Legendary powered game. I wouldn't allow any magus to bind a familiar of that might unless another character wanted to play that familiar (note, I wouldn't force them to give up their magus to do it).
Beast of virtue tend to be 9-25 in might, and I wouldn't allow any creature to go much beyond that for binding purposes.
If a player wants a small familiar, I don't have a problem with it. The familiar might never leave the lab now. Heck, one of my characters has a familiar that prefers sitting at home while his magus goes out on adventures. He's no dummy, it's much safer in the lab...
Bear in mind the whole thing with Might and your character's role refers to base Might, not end Might. So you could have a Might 60 Legendary-saga Companion if you got 10 (or more?) Inferiorities and then spent 35 points on Improved Might.
I was personally planning on an Anthill as a familiar if I can persuade my SG.
It'd be the queen ant as the familiar, and she'd have some powers to spawn the rest of the ants, as they are creations of hers through magic. I was going to use the size of the swarm combined to decide the creatures size, however. Seems more reasonable than -15 or so.
Nah, that's not abusive at all.
Well, what size is that? Ant colonies can be quite large. Is that size 0 or size -5? And what might level are you thinking?
I can't tell if you're being sarcastic or not (I imagine you are, as that method is still pretty munchkin-y).
As for the OP, small things tend to be super easy to kill, so I would suggest some sort of power to increase Soak, or at least manifest a body that's less likely to die from a stiff breeze.
Apart from that, Using the rules for RoP:M, I would argue 25 is the absolute upper limit on Might for the creature, at least Base Might.
As for the actual animal, a sparrow, or a ferret is smallest I'd go (Size -5 or -6), but even that leads to them being stupidly easy to kill. Size -4 (cat) at least allows some wiggle room. If the player really wanted to have a familiar that small, I'd strongly suggest they make it more resistant to damage as a first and foremost priority.
I left it up to the reader to infer, true. But, yes, I was being sarcastic, when I'd already outlined the idea that a creature with such a high might score needs to be played by another player, and it's certainly only appropriate to certain kinds of sagas. More might doesn't necessarily grant anything except more magic resistance, which is probably the least of the familiar's concerns, especially a small familiar that can be squashed like a bug...
I dunno if I'd call that munchkin-y or abusive. You just spent all of your Might-granted Virtues and ten points of Inferiorities on raising your Might... Which still leaves you with no Powers, the ability to speak, or really anything except your mundane statistics and a high Might score. Granted, at my table, player characters with Might are only allowed to have as many Inferiorities as a character of that type could have Flaws, so... (I don't remember if that's a House Rule or an actual rule, Serf's Parma) Such a character would basically be in a saga surrounded by extremely powerful, probably old, and most likely resource-heavy magi (I mean, it's called Legendary for a reason) and basically have nothing going for it except for being able to resist non-specialist spells. (Not that being able to resist the CrIg of a CrMe specialist isn't useful, but come on.)
House Rule. I don't think such a rule exists in RoP:M. I honestly prefer to work with small amounts of Might (even as low as 2 or 5), so that you can strategically pick up Inferiorites and create a flavoursome character, as opposed to some butterfly with 50 Might than can talk, and do little else (apart from being a preternatural tether of strength 5).
RoP:M, like so many rules, are really more like guidelines.
Just because I can design something doesn't mean I can have it. That includes perfectly legal spells. So a GM who eliminates all teleport spells because he wants travel to take longer is completely fine.
I can design a Might 3999 creature. But does it exist? (Let's leave Saint Anselm out of this!)
I can also design a Might 5 being that looks like ET. But does it exist?
So, the GM and maybe group get to decide, "Does the Might 60 Thing exist? Do I want it in my world?" And very likely, a Might 60 butterfly will not exist because with great power comes great everything else, and butterfly is just too puny to hold 12 pawns of vis. But if you pass that, he might say, "Ok, this does exist, but you need to convince me that this godlike being wants to be so intimate with you. And then, if you pass that, he might say, "So, you're his minion, right?"
I like powerful familiars, for the record. I haven't gotten beyond the 40s, though, which is still firmly in powerful angel territory. But these guys have agendas of their own, and the magus better be compatible with it. So in a high level game that someone tried to run here, for example, I gave my character, Deborah, a powerful spirit associated with insects. It was a good match, because Deborah was into vermin. But Deborah was my AM Shadowrun Insect Shaman, or close to it. She was not in command. She was no longer fully human, nor were her goals. Aaaand, in most games this character would be denied.
Just saying.
You know... I had missed the sarcasm completely. Personally, I don't think it's particularly abusive. We had a Might 50 familiar (a vastly powerful spirit) in one of our sagas!
I do, however, agree with Ovarwa on the fact that butterflies with Magic Might 60 probably not exist at all. Remember that magic beings with Might above 75 are fundamental building blocks of reality like Love and Sky and Night. Between level 50 and level 75 you find the Kosmokrators / Titans, the great spirits of primeval places like the original Hyercinian Forest or the great Tengri, the greatest dragons of legend (with Jormungandr, the Midgard serpent than encircles the world, being among the most powerful of these).
Are there any butterflies in this range?
The greatest (magic) jinni lords reach Might 40. The spirits of the Planets have Might 40. The fragments of the Hyercinian Forest have Might 10-50. Typical dragons are Might 30-40. I would say that the Queen of All Butterflies may have Magic Might 30-40 at most.
A creature with high might isn't abusive but taking improved might 35 times probably is.
grin For true and utter abuse, if there is a demand that a familiar have a Might of no more than 15, just start off with Might of N vastly greater than 15, then take the negative quality of reduced Might N-15 times, buy goodies to balance, and voila.
Essentially the counter-point to the previous argument 
You know, I had forgotten about that. You make a good argument. I can imagine a random magical butterfly eventually self-transforming into Might 40, but at that point, it can barely stay in the corporeal realm any more, and will probably spend most of its time in the Magic Realm.
That actually brings a solution to all of this to mind.The character picks a broad range of Mights (0-5, 5-10, 11-20, 21-30 etc.), with each category requiring the character to be more convincing and the familiar to be more attuned to the character. The SG picks the actual Might of the familiar, but the character can decide on V/F and Q/I (with your house rule on limiting the number of Inferiorites if you'd like). You can fluff this out as the character deciding what he wants in a familiar; the more specific he is, the harder it will be to find. Of course, the SG has final veto power. That has worked so far for me, but all of my players are new to AM, so they don't really go for abusive options.
Wait, you mean normal sagas don't make it progressively harder to find a good Familiar as the Might and prowess of the being increase? Seriously, does anybody remember that high-Might creatures are actually supposed to be pretty rare? Finding a suitable Might 50 Familiar should, at the least, take over a year and involve multiple stories. First you actually need to locate an applicably powerful creature who isn't of the "fight you for entertainment" variety (unless you have some nonlethal physical version of the Tytalus mindset, but even then a sporting battle isn't really a conflict per se) which is a challenge in and of itself. Then you need to go there and the usual requirements of bonding come into play, which are both slightly harder than normal as the being in question likely looks down on you significantly, and more importantly is difficult because you basically have one creature to pick from. So it probably doesn't work out, at least not the first couple of times.
Generally speaking, the lower the Might they try to go for, the more variety of options they'll have, and the more likely they are to fairly quickly find one that bonds well with them. What was that, you want a Might 5 rabbit? Yeah, sure, you can even make it yourself, there are plenty of Might 5 bunnies teeming about. You want a Might 45 great dragon? I'll draft up a couple for you to find, let's hope you like one and can get it to like you.
I may be a bit extreme, but do most games not make finding high-Might Familiars harder?
I'm new to the whole Ars Magica storyguide thing, so I don't really have a reply here. I can reason that higher Might creatures are rarer, hence the above statements, but I don't really have the experience to say whether most people do it that way. 