wheel and tyre sizes

 
I assume you are looking for a softer ride.
With an MR2 because of the rear weight bias, for balanced handling you need staggered sizes with wider rears than the fronts. (one inch difference is normal)
 
For normal street driving with 16" wheels the sizes often used are:
Front 16x6 or 16x7 with 205/45R16 tyre
Rear 16x7 or 16x8  with 225/45R16 tyre
 
for 17" wheels something similar would apply except that you would need 40 profile tyres to maintain the right rolling diameter.