hmm, picture garbage cans laying down with a layer of rubber bands wrapped around them. Thats about the size j/k. 10's in the rear and 8.5's or 9's up front i would think. Its a lot to do with offset aswell since you want the outter lip to fill but not rub, and you dont want your inner lip to rub your suspension or brakes. To spec out the size you need you have to measure from your hub mounting pad to your desired outboard flange or where you want your wheel to end on the outter side of the fender. Offset is measured by subtracting the backspace from your centerline of the rim. Search around for wheel spacing caculators and such, since having the wheel fill the flare propperly is what really 'makes' the flare look good and create functionality. good luck, and your flare fab looks super sick- cant wait to see it finished.