Two main quetsions:
If you are giant sized (+2) do you need to increase the target to group?
Quoting the text, "Increase one of a person's physical characteristics by one point, to no more than +5." Does this mean you can invent one spell of this type and use it on Str, Dex, Qik, and Sta, or do you have to invent a spell for each stat?
Either target group or add a magnitude for size. The usual way is adding a size modifier.
One for each stat. See "Gift of Reason" for an example.
I totally agree with ulf.
Just one question. If you researched a CrCo spell, which increases you Quickness to +0 (with subsequent castings), and you want to invent a spell that does the same for Stamina, would you consider this as a similar spell and add the spell's magnitude to the labtotal? I would rule no, but we had a discussion about this in our group recently.
I would also agree whole heartedly with Ulf, and would say that the spell to affect a giant blooded persion would have to have a +1 size modifier.
As for spells being similar when they increase different stats, as long as they are the same form/tech (i.e. no similarity between using corpus and mentem) and as long as they are boosting the stat by the same amount to the same limit then yes, they are similar. If the only difference is one works for strength and the other for stamina, thats certainly similar.
The first question is covered on page 113 of the core rules (Targets and Sizes guideline box). In fact it specifically mentions adding 1 magnitude to a spell/effect to affect a giant up to 15 feet tall (which is size +2 actually).
As for the issue of spell similarity, its quite clear from page 101 (similar spells section) that a "similar" spell is one which either:
has the same effect at a different range, duration or target level (all three may even be different) or...
Closely related effect at the same range, duration or target level.
In the case of the second, I don't much like the example given in the final paragraph (CrIg somehow being closely related to a MuCo(An) spell??, don't think so) so for continuity I would suggest that the form and technique would have to be the same at the very least for the spells to be considered closely related.
A CrCo Quickness enhancer would most assuredly be closely related to any other CrCo characteristic enhancing spell/effect so I see no reason why you couldn't benefit from the added lab bonus for spell magnitude.
They wouldnt need to boost to the same amount or limit, no, Gribble. The core rules citation above makes that quite clear. I would argue that under option #1 (above) "same effect" would equate to "physical characteristic" boost not necessarily and specifically "stamina" or "quickness" boost. Thus all CrCo effect/spells to boost any characteristic by however much to whatever limit would count as "similar".
removed due to being pointless after seeing the post above...
Spell size's and creature size's aren't the same thing. A 15 foot tall giant is probably size +4. But it's still only an increase to the spell of 1 magnitude.
This is an argument that has cropped up time and again in our group. Fact is spells don't have "sizes" they have magnitudes and levels. "Size" is always a reference to the target being affected in the Ars Magica system. I suggest you go back and read the pages mentioned (along with the size paragraph for character creation on page 13 and the animal size chart on page 192). You are however correct that a 1 magnitude increase to a spell allows for a target size effect of +4 (when dealing with Corpus/Animal base form sizes).
You also have the right of it in correcting my +2 reference. In point of fact, 15 feet tall IS +4 (elephant sized) and not +2 (which is the size of characters with the giant blood virtue) and thus around 8 feet tall.
I think you misunderstand the example. It isn't comparing Creo Ignem damage dealing spells, and spells to turn people into land animals. It's saying that those are two example categories of similar spells. Creo Ignem damage spells are similar to each other. And in much the same way, spells to turn someone into a land animal are similar to each other, though each spell may change the target into a different land animal.
Ah, yes. So much for rereading references whilst typing. I'll add that to walking and chewing gum for mutually exclusive activities. lol