Wheel Size

[Gad]

Just purchased a mondeo ST 24 (had to sell the tvr two kids now!!!!!) on a V plate does anybody have info on the maximum wheel size that can be fitted without any modifications ie 215/40/18 i would like 18 inch rims with 215 or 205 tyres but not sure of profile 35 or 40 although the standard wheels are nice 4 spoke they don't fill the arches so well thanks.
Gad