yeah, as mentioned above 19" is the biggest you can put on a celica...i've never seen/heard of a celica w/20" as of yet.
most people who are "go" oriented usually go with 16s or 17s (some, more serious racers will choose 15s) while most people who are "show" oriented will go 18s - 19s. (if you go 18" or 19" be sure to keep your tranny in excellent condition. these rims add a lot more strain to your drivetrain).
if you go 18" or 19" you will need super low profile tires Z(obviously) in order to fit them into your wheel wells. as far as width of wheels is concerned, most people tend to go 7.5 or 8. the exception being those with Fender
flares and opt to go with wider wheels instead of tires.
some people have also gone 8.5 - 9 in the rear. this is totally cosmetic (this is to make your car look like a rwd setup) seeing the celica is fwd.