Wheel sizes are best complimented with the right ride height, for example...
A base spec 156 with 17's will look abit tall there by making the wheels look smaller that they are.
A sportier spec 156 should have a slightly lower ride height making a 17" wheel look better.
Also another thing to take into account is wheel design, some designs ie split rims or spokes that only go to a dish will look smaller than they are, a wheel with a spoke that goes right out to the rim will look bigger.
17" are probally the best trade off between handling and looks but Im sure you could push 18"s and still live with a slightly ruff ride.
What size wheels are you running at the moment?
ALFA ROMEO 156 TS & ABARTH 500