Assumptions :
- A large tree is size +3
- something small enough to fit in a pocket is size -4 (assuming large pockets)
That's a difference of 8 sizes (counting size 0)
Enlarging and shrinking spells always are hard to figure out but thanks to caribet, he provide us with a little something to easly calculate sizes and masses.
We can see it's easier to actually shrink than to grow (as seen in muto animal spells)
The base of the spell is then 4 to reduce 2 sizes.
Now we need to simply add one magnitude for each other two size decrement
Assume it's a touch spell and you want it to stay reduced for a sun duration
4 base, +1 touch, +2 sun +3 size
So it gives a spell lvl30 in muto herbam
Seems ok by me, hope it was usefull !