I'm confused by the rules for growth. Both animal and corpus have spells that increase the size by +1 (roughly 3x the mass) at levels 4 and 3 respectively, whereas Terram level 4 increases the mass by 8x, which seems oddly arbitrary. Are there any RAW about increasing size past the listed amount? How many magnitudes to make a small box the size of hut? What about making a dragonfly the size of an elephant with one spell?
I generally suggest each +1 magnitude past the basic effect can apply the basic effect again. So, for humans and animals every +1 magnitude add another +1 to Size (if that's the base spell guideline; can't remember), and for rocks every +1 magnitude multiplies mass again by x 8.
The only complication I'd add is that you need to be able to affect the larger Size with your spell. So you will probably need to add further magnitudes to increase the size of the Target of the spell, as the Base Individual for it will be too small. Note that each +1 magnitude adds x10 to the mass of the Target, so the calculation is different from above.
As a house rule, I'd suggest simplifying all these size/mass calculations to have ALL base effects (in all Forms) increase the size by +1 Size (about x2 mass) or by -2 Size (about /5 mass), with each extra magnitude allowing another +1 Size (with +3 Size being x10 mass) or -2 Size, and with Target sizes likewise increasing at +1 Size per extra magnitude. This house rule makes things more consistent and simple, and makes affecting large things harder (which I think is a good thing). But this isn't the RAW.
An increase of +1 size is closer to twice the mass(10^(1/3) = 2.15). I think the idea of the Terram increase was to approximate doubling the length, width, and height of an object, which would indeed increase it by 8x the mass. However, it perhaps more simple to consider this to be an increase of 3 points of size, and thus increasing the mass by 10x.
I add a size modifier (1 magnitude) for each 3 points of size, rationalising this as requiring the spell to match the beginning and modified size of the target affected. This is the rule used in Magi of Hermes (Chapter 5: Hugh Hostis), where the spells Assume the Stature of the Giants of Eld and Turb of Giants both have a Base of 3, a Size modifier, and add 3 to the Size of the target/s*. Furthermore, in Chapter 6 of MoH, Julia's loom can be shrunk to a tenth of its former size in each dimension (a thousand-fold reduction in mass), and the spell text says "This spell is designed based on the Object of Increased Size spell (ArM5, page 154), but has an extra magnitude of complexity to make the loom shrink to a smaller size than allowed by that spell."
*technically, Size +4 is possible for this spell, since the base individual for Corpus is Size +1, so the base target with a Size modifier is Size +4. However, I preferred the symmetry and simplicity of a +3 increase for all, so Size 0 become +3 and Size +1 become Size +4.
What I've done in the past is use the rule that 1 magnitude represents one step on the size table in RoP:M. This actually fits pretty much all the spells that have anything to do with size changing - at least within the Animal and Corpus set.
So if you have access to that book, that's a handy guideline.