Widest rims that will fit in the guards, with smallest diameter that will fit over your brakes - less mass and tyres are cheaper in smaller diameters.

Narrow (i.e. stock 6") rims make sidewalls feel floppy, go wider before you band-aid it and go bigger. Big rims and tyres add much sprung weight, you'll hate it on a light car.

Gearing is usually not a factor since you can change the tyre profile to keep the same rolling diameter.