I have a nice set of stock 2002 MR2 Spyder wheels and tires on my 87 MKI. I picked them up as brand new dealer take-offs (somebody upgraded before taking delivery) for $500.
The tires are Bridgestone Potenzas and the sizes are: 185/55R15 front tires, 205/50R15 rear tires, alloy wheels. The rear wheels are 1/2 inch wider than the fronts. I'm remembering this as 6" front and 6.5 rear. They are a perfect fit as to offset- no rubbing and they look great. They are only a couple of pounds each heavier that the stock alloys and tires they replaced and there is a lot more grip. With these tires and stock suspension the car understeers more than it should at full throttle. There is no anti-sway bar on the back and I'm hoping to balance the handling when I install new front and rear bars.
After 2002 the factory stock set up was 185/55R15 front tires, 215/45R16 rear tires, alloy wheels.
I think a 2cm difference between the front and rear tires is plenty (if not too much) to match the difference in the the static weight load. I would warn against tire/wheel combinations that add a lot of unsprung weight to the suspension.
The original tire size was 185/60/14 front and back on 5.5" alloy wheels (I think SC's got 6" wheels). Tire Rack suggests plus sizes of 195/50/15 or 195/45/16 or 205/40/17 for a similar rolling diameter. Anything wider than that is going to raise the center of gravity.
Just for reference the standard tires for a Lotus Exige S are:195/50/16 front and 225/45/17 rear. This is the same as the Sport option for the Lotus Elise and are mounted on 16x6.5 and 17x7.5 wheels. I mention this only because the Lotus is a lightweight, rear engine high performance sports car. The Lotus wheels have the same bolt pattern as our MR2's but the center bore is slightly larger.