How big do you want to go? 17, 18, 19 (rim sizes) have all been done. I'm sure someone has put a 20 on a stang somewhere. as far as width is concerned I wouldn't go any bigger than a 10 inch rim, 305mm tire.
I would stick with 17 or 18 inch rims. The width is up to you. You could stick with a 9 inch all around or a 9 in front and 10 in back but then you cannot rotate tires. My setup is a 18x9 rims, 265 35 18 all around. Hope it helps.
Toohey