So if I cast it to make a size -6 newt Size -5 with Sun duration, can I keep casting it till I have a Size +10 newt? 
cj x
I don't believe so. Like all other Size enhancement spells I can think of, the improvement is based off of the target's original size.
First, even if it can stack, not you cannot get to Size +10. Eventually the target is too big and requires extra magnitudes for T: Individual to be built into the spell.
Second, as a general rule I would never allow stacking of the same thing. That leads to all sorts of problems. I feel this should just be a single, much higher magnitude spell.
Chris
This.
Not sure about spells stacking, but once the owl passes size +1, that takes a size modifier
Yes, we took that in to account 
What I'm unsure about is whether it stacks. Reason being I can't find anything in any of the Size change guidelines or spells that suggests otherwise? Had never occurred to me till it came up in my IRC game.
cj x
Consider something else listed in the guidelines instead. Let's go with MuCo and Soak. Why are there so many guidelines? Why not just make a +1 Soak version and cast it several times before venturing out for the day instead of wasting time inventing a more powerful spell? My answer would be that they don't stack.
Chris
I don't think it's ever been spelled out, but the system does appear to assume that stacking is not an option.
In my opinion the vagueness about stacking goes even further.
Maga A has a shrinking spell that turns an Individual human into Size -4. Magus B has a growing spell that turns an Individual human into Size +2.
Alice is a mundane servant who dreads getting up in the morning because she lives with Maga A and Magus B.
What happens if Maga A casts her shrinking spell upon Alice, and then Magus B casts his growing spell upon the shrunk Alice? What Size is she at the end? Then what happens if someone dispels Maga A's spell but not Magus B's?
Personally, I tend to prefer things that aren't just about the same stacking, even when they affect the same thing. So, for instance, Gift of the Bear's Fortitude still provides its bonus against a fire strong enough to get through Ward Against Heat and Flames. So in this case I would say -4+2=-2 or +2-4=-2 (so it doesn't matter), though the latter may not work if the spell is not designed to affect someone of Size +2.
Chris
IIRC the spell acts on the original size of the target,
So Alice (size +0) is shrunk to size -4. She is then affected by the growing spell, and becomes size+2.
A size-6 newt raised to size-5 cannot be affected by a boost-1 spell, but a boost-2 spell would make it size-4. Similarly a boost-12 spell would take it to size+6, and if then hit with a shrinking-4 would reduce it to size-10.
Which spell has priority is debatable. For example, i would assume that a +6 boost with 20 penetration would be overidden by a +5 boost with 21 penetration. (Penetration meaning excess casting total).
Presumably a Sun duration effect which has been overidden by a diameter effect will come back into force afterwards.
Make sense to anyone else, or am I assigning logic to a fantasy game again? 
K.
I think there are several plausible systems for determining the final size - I count three at least in the two previous posts! - but I think the core rules are agnostic among them.
I also think any plausible system for determining the joint effect of a growing spell and a shrinking spell should also treat multiple growing spells, multiple shrinking spells, and indeed arbitrary mixtures.
What about the idea that two "same-direction" spells have the same effect as the more extreme of the two spells, while two "different-direction" spells sum their effects? So a boost-2 together with a boost-3 would just boost by 3, and a boost-3 together with a shrink-9 would end up shrinking by 6.
But then, what would be the outcome of a boost-2, boost-3, and shrink-9 all together? A shrink-6 (first choosing the max boost), or a shrink-4? ... 
This is what I was suggesting. It's also very consistent with the behavior of Characteristics when shapeshifting, and as that's Muto it seems reasonable.
I would point out that these are multiple ongoing effects, so if you follow your comment above the boost-2 is overrun by the boost-3, regardless of order. That would result in shrink-6 regardless of order. It would only change if you forget that there are multiple ongoing effects and start considering them to be only one effect.
Chris