6x15's (Deep 6's) fit with no problem with 185/60/15's
remember you'll also need the hub/discs converting to 5-stud, I've got some handy tools for the job if you need to borrow them, nothing technical just a hub puller and bit of scaffolding for the castle nut.
Any bigger with the wheels and you may have to 'tweak' the arches, Kevster has some bigger wheels on his car, hopefully he'll chip in