17s are pretty much stock. Anything under that, like 16s or 15s (slicks) will yield you with more traction but worse handeling. Anything OVER 17s will yield better handleing but worse traction.
19" rims will also throw off your speedo, so you will need to get a tuner. Personally 17s are good, but 18s look the BEST on 99+ stangs. 19s and above are too much for my taste.
Also, with bigger wheels comes greater weight. "Roatation mass weight" (ie: heavy wheels) is the worst kind of weight you can have in the car. Heavy 19s may slow down your car a lot, you might even feel the car get slower. And vice-versa: going with light 15" wheels may speed your car up. Its because there is less diameter in which the tires have to cover to complete a rotation, and the raw weight of the wheels is an added stress.
Many people on this forum have 18s, 19s, even maybe 1 or 2 with some 20"s...so it can be done. But I just wanted to warn you about having bigger wheels. I wouldn't go any bigger thna 18s, personally.