You can go as big as you want, but the dtrimentbof handling and speed. 16inch rims with 225.45.16 is fine and looks good while still being safe for the rims. You could go to 17inch rims with 225.40.17, but the shoulder is become stiffer and smaller. If you check www.e30.de you will find people with 17 x 9 rims with 245 rear tires, but it is becoming very cumbersome. Do not go beyond 17 X 8.5 with 225.40.17 with ET 20.