Bridgestone Potenza RE050A Specs and Review

Namir

Performance wise, the smallest wheel you can fit while still clearing the brake calipers will be the 'best' because it'll always be lighter than the exact same wheel in a larger size.
Assuming you are running dedicated track tires, they should be light and have minimal sidewall flex so there is no reason to run a larger diameter wheel.
Most guys who track their cars have 18's or smaller. 19 and 20 inch wheels are usually just for show.

Of course if you can afford 2000$ per wheel, you can get super light forged 19's that are both light and sexy...