Rims Size: 16-19in rims have been use don the Contour. The 18 and 19 can take some heavy modification to work. 17's are my personal pic based on looks and performace. Big rims are usually heavier and sap more HP.
Rims Width: I would stick with a 7-7.5" rim width. 8" has been done but usually rub's pretty hard.
Tire Size: My personal belief is that 225/50 is about as big as you want to go, and that size will only work with certain tires. IE 225/50 Firehawk SZ50 EP's are fairly thin for 225's. BNoon has done 235's.
Lug Conversion: As far as I know, it has not been done and in most cases wouldn't be worth your time.
-Andy
