Hi Stegg,
It's really up to you but I wouldn't recommend going larger than 15" unless you are building a "show car" rather than one to drive on the street.
Having really large wheels may look good but can actually spoil the performance, ride and handling of your car. It may also be illegal in your country to go larger than a certain size.
In Queensland it's illegal to fit an aftermarket wheel/tyre combination that is more than 15mm larger in diameter than the original factory rim/tyres (155/70R13).
This means they most common aftermarket sizes used here are ;
14" rims with 185/55R14 tyres (12mm larger than standard)
15" rims with 195/45R15 tyres (9mm larger than standard)
Good luck.