In another age, we discussed this a bit:
https://forum.atlas-games.com/t/size-0-to-2/349/1
W
Incidentally, I'd point out that assuming that the spell must be able to affect the final form of the Target is incompatible with Preternatural Growth and Shrinking, which can turn a size +1 human into a size +2 human!
Yees, that bothered me as well.
And no, I still haven't located where I saw it the last time. 
Having seen a grugach in play using this kind of effect heavily on grogs in a recent minisaga we have seen how powerful it can be. Gruagachan have the additional effect that equipment and the like do not imply extra magnitudes/requisites, so they are even better at this, but the general premise is that size modifiers are needed for larger sizes. Base 3 is useful for a size increase up to +1. You need a +1 magnitude for size +2, +2 magnitudes for sixe +3... So the scale ends up being like this:
B3. Increase size up to +1
B4: increase size up to +2
B5: increase size up to +3
B10: increase size up to +4
B15: increase size up to +5
B20: increase size up to +6
Add range, duration and requisites for equipment on top of that.
Ogre sized turbs or above are quite something if you just want killer power, even if they are not easy to cast: Touch, Sun individual is MuCo30 adding Metal and Herbam requisites to change a grog to size +3, so going for larger grog groups tends to cause warping.
Cheers,
Xavi