Like all around street driving or serious track business?
The size isn't going to effect handling as much as tire width and compound. A smaller wheel with a very wide sticky tire will out handle a larger wheel with a cheapo tire.
A 17" wheel will cost less to buy tires for than an 18" so if you are on a budget you could get a better performing tire for the same money and thus better handling. If you're not racing buy the wheel you like the look of cause that is what most people buy them for.
I believe from the auto-x guys one of the best tire sizes is a 235/40. It provides the largest contact patch with the lowest sidewall. Your speedo may be slightly off but if you are looking for "the best" handling tire size, 235/40 in a 17"
Popular tires for serious grip: Dunlop Direzza Z1 star specs, Bridgestone RE-01s, Hankook Ventus R-S3.Again though serious grip and soft tires means they will not last long. Like 12k miles instead of 30k.(obviously depends how you drive)