Anything larger than required to clear the brake rotors will do very little, if anything, to improve handling, and unless you spend a ton on lightweight forged wheels you will likely further decrease the handling. If you are looking for show, then get big monster wheels, but then the tiny brake rotors/drums look just plain silly so you'll have to swap them out too. Then you are left with the covered wagon look... Anything over 15" is overkill, ride quality, handling, acceleration, and braking will likely suffer. How much do those wheels weigh?

/bill