Splitting out for discussion. This does need to be clarified.
Looking at the general size rules, we should probably be working with factors of ten, based on mass/volume.
In that case, if you have a base guideline for growth by a factor of 10, and adding another magnitude allows growth by a further factor of 10, you naturally take account of the size, based on the final product. Thus, there is no need for further adjustments for the size of the result, as there might be if growth were faster.
Going the other way, there is a question of whether we want it to be easier. If we do, then one magnitude for a factor of 100 is probably the way to go, with a base of 100. The MoH spell would need +2 magnitudes, and the HP one would need +1.
This would involve errata on the MuTe guideline, as well.
That's what was done with growth in MuCo, though the original is +1 Size rather than +3 Size. But for additional growth an increase in size handled it.
That would be your choice first. Yes, MuCo shrinking gets -2 Size as easily as +1 Size, so there would be some consistency there. It is interesting to note that shrinking needs magnitudes for more size separately from magnitudes for more shrinking if we look at the HP spell, while growth doesn't seem to. But if shrinking gets that factor of 100 instead of 10, there is some fairness there. It's still a little confusing that growth gets to merge them, but we've lived with it so far.
MoH. Now, there is a question about the size of a grain of wheat since they vary in area by a factor of about 10. Unfortunately, standardized grains and wheat grains are units of mass rather than volume. Looking at volume, they're typically about 5 to 9 mm in length and about half that along the other two dimensions. Assuming a spheroid for simplicity, this would give 131 to 763 mm3. So 450ish mm3 is a good ballpark. The chest has a volume of 5.625 ft3 = 160 000 000 mm3. So the maximum shrinking is by a factor of about 354 000. 100 x 100 x 100 handles that, so yes, +2 magnitudes would work.
HP. This is simpler. It's reduced by 2700, so yes, 100 x 100 is enough and +1 magnitude would work.
I'm confused. First, are you talking about growth or shrinking. Second, what do you mean by "magnitude 2" for "10 times 10"? The base magnitude is given by the base, which can vary by Form. The final magnitude will vary from there based on what's been included for R/D/T, size, and maybe other stuff.
Ah, OK. Here the base includes the first x10 because that's what the base is doing. David Chart said to be consistent with what you found it would be good for it to be x10 rather than x8 (the minimum required for the spell). Each extra magnitude added on allows another x10. That's why x100 here is just +1 magnitude added on, as it's only one step bigger than the base.
That is what I am saying, but it is not clear from the published rules — hence the discussion of possible errata. We do want to make sure that the clarifications actually make things clear.
It may cause some errata for Dies Irae but I'm not sure.
I do remember spending a lot of time figuring out exactly how size changes appear to work in the current version though, back in playtesting.
I propose adding this, nominally to the insert on p.113, because the discussion of Targets and Sizes seems like the best place to handle this.
"When changing the size of the 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."
Corpus and Animal usually use a much smaller scale, so making a blanket statement like this, as much as I like it overall, may be problematic. What you've got there says +3 Size or -6 Size fit in the base, while MuCo/An allow for much less.