16s will be fine as long as your using 40 profile tyres and the car isnt way too
low! 15s look pretty decent too.
All depends on how your want your car, if you have a kit on it then 15s will
look tiny.
If you have just lowered it then 15s will look fine.
16s wont look out of place either way imo, but then tyres are more expensive
and you want get the nice feel you get with 15s...