I Find myself wanting to invent a version of the spell "Beast of outlandish size" that increases size by more than +1.
Specifically I want to transform my Size -2 familiar into a huge beast, or as huge as my MuAn lab total will allow.
I am having some trouble figuring out how this is done.
Initially I thought it was just a matter of increasing the magnitude of the spell, but it seems to me more likely that magnitude concerns the maximum size of animal that can be increased in size by +1. i.e. the base spell increases size by +1 and can be cast on an animal no larger than size +1. Increasing the magnitude under this interpretation simply makes it so that the maximum size of the target is increased.
What guidelines apply for increasing the size of an animal by more than one step?
Unless you see a rule or altered guideline somewhere I think it may be +2 (in addition to the initial +1) to the being’s size per added Size magnitude of the spell. Least that’s what seems to be the case with the one spell I found that does this, Turb of Giants on p49 of MoH.
The only thing I can find is the errata which states that:
"When changing the size of a non-living target, the base guideline is for a spell that enlarges a target of the base size by a factor of up to ten, or shrinks it by a factor of up to 100. Adding a magnitude to the guideline allows the spell to enlarge the target by a further factor of ten (for a total of 100 times larger), or shrink it by a further factor of 100 (for a total of 10,000 times smaller). Modifications for the size of the original target are in addition to these modifiers, so that a spell to shrink an individual ten times larger than a base Individual by a factor of 10,000 would be two magnitudes higher than the guideline given for changing the size of the target. Forms dealing with living targets have their own guidelines."
It then does not state what the guidelines are for Forms that deal with living targets.
What you state is exactly the errata'ed guideline for non-living targets.
I cannot find anywhere in the core rulebook where these supposed guidelines for increasing the size of living targets are, that the errata refers to.