Are you asking what are good sizes? like for handling/looks? or do you just wanna know the maximum size you can fit in the fender wells?
With the proper offsets, wtth stock suspension, I've seen 8" in front and 10" in rear
If you're looking for a nice flush fit, most MR2 owners go with something similar to this:
Front 8" wide +35 offset
Rear 9" wide +35 offset
Slight variations can be made of course, but this will bring the rims flush with the fenders. The only thing is, for this to fit, the rear fender lips will have to be rolled.
If you're wondering, my car has the following
Enkei RP-02 rims
Kuhmo Ecsta MX tires
F 17x8 +40 215/40/17
R 17x9 +35 245/40/17
Hope that helps. Best of luck with your new 2!
Frank