standard rim is 5". Be careful with offset as same offest i.e 38mm on a 6.5" compared to 5" means the wheel sits out half of 1.5" (i.e .75" ~ 20mm) further, which can cause scrapping on the guards etc. So in wider rim you may have to look for larger offset, but then you have to be careful about clearance with shock tower etc. I'd try not to go bigger that a 6" rim.
I had problems with fouling on guards running 6.5" 35mm offset rims with 195/50s fitted (AE82 seca)