I just put new speakers in my 89 supra turbo, and the front and back speakers were 4", and I had 2 middle speakers that were 3 1/2". I put in some new 4" infinity kappa speakers and did not even put in the 3 1/2" speakers. I also got an infinity amp and sub. It sounds great.
I do know that not all the supras have the same size speakers in the back, but I,m pretty sure that your year of supra has 4" speakers in the doors.
For one, you can just take off the door panel and measure for yourself or you can look in the manual, or just go to bestbuy or circuit city and go into their registry. As for the speakers themselves......it's all in what you want to spend. Sony explodes sound good for the money, as well does Polks. But might I suggest Kicker Comps for quality sound, however they are more expensive. Most quality speakers these days require an amp to power them, unless you want the speakers to blow.