I was thinking size -3 or so. And probably somewhere between might 15 and 25, depending on what the powers end up needing. I'd probably work out what the powers need for each possible might level so I can work out which is the most useful.
Edit: As for a might 40+ butterfly. It's a butterfly of storms. Somewhere, it flaps it's wings, and then there's a storm or a hurricane.
I'm kind-of with Lucky Mage on this one.
If someone really has a great vision for a character with a might 60 butterfly as a familiar, I have no problem with that concept. I'm even willing to work with that player on exactly what this butterfly is and why it's might 60.
However simply writing it on a character sheet doesn't mean the character arrives at the covenant the next day saying 'Hi, I want to be a familiar! Any takers?'
There's a story to be told there. The player has a reason to pursue the story. This sounds like a good combination of things for me.
I don't know, it seems like one could readily enough invent a CrAn(Vi) spell to make a Might 60 butterfly.
Readily, in the sense of pulling together a 220 lab total for one season, yes.
That's a lot of vis in your ritual. If your SG is running with the rule interpretation that all rituals are stressful...try not to botch.
Well, the risk of botching might be less than the risk of adventure...
I would give them a fairly small might. Even if it was the God of all Mice I would make it way bigger than a standard mouse. Other than that give it appropriate powers. A mouse can probably burrow through anything and destroy plant products for example. Or it could be like a fire butterfly which makes pretty little (but deadly) flames.
Point is it doesn't really need anything special other than an awareness that you probably will have a vastly higher lab total than you need.