i beleive 14's are the smallest,
i hear that you can grind your calipers but i would roll 14's,13's are
just a little to small.then if you punch it,your motor will rev kinda loud.
a buddy of min has tried 13's.
i got a '94 caprice ex-cop car, they just like the impalas, same engine and stuff. i grinded down the calipers and roll 13s, and when i mash on the gas the engine doesn't that much louder than with the stocks.