I saw this thread once before on these forums and I think the consesus was that 225's are the widest you can go.
Depends on the year of the car as well, I think I believe the early model celicas had narrower (closer together) body rails. Then they changed to the wider body (RA23 style, wider guards) before they moved the body rails out.
If you have a celica with narrow body rails and the wider body/guards, then I think you can go bigger then 225's.
Correct me if I'm worng anyone, it's just something that I remember reading somewhere and have no idea about how accurate it is.