Tire Size?
 

I just picked up a set of 3rd gen rx-7 rims, they are 16x8. What would be the ideal tire size to keep handling the same or better?

offset? is it 16x8 all around? or just for front or rear?

Do RX-7 rims fit an S2000? Same bolt pattern? How about offset?
In any case, an 8" rim may be too wide to put on the appropriate sized front tire for this car. Some owners have gone with a 225 up front and this will be OK for an 8" rim. Then you can go with a 245 or maybe even a 255 for the rear. None of these will be "ideal". This is all assuming that the offset on those rims are adequate for S2000 application.
Having the same width of rim on all 4 corners is not ideal, but will work. It is doubtful that you will find any combination that will be as good (let alone better) than OEM sizing with SO2s.

245/45/16 is what im runnin on my 16x8 rear

Is OK. 255 might be a bit of a stretch but still "OK".