I didn't want to clog the "18s on a Z" thread. This thread is "mine" so i will put this sort of philosophical post here
The top pic is a Varis Supreme Widebody Supra Mk. IV. It's a car 15 years old, but in its time, it was a halo car that represented the peak of japanese sports car. Even with VERY aggressive aero . . . mere 18's look fine. I have no complaints. Visually, the car is pretty balanced.
Why? Because back then, the wheel wells weren't cartoonishly enlarged, they were designed with 18's in mind, so even with lots of stuff slapped onto the body, they still work. And that's a show car.
Contrast with the 2nd and 3rd pix, Amuse 370Z's. The middle car has 19's and STILL looks like the wheel are too small. The Amuse kit extends down, so but you can still see how proportionally, the wheel wells LOOK so much bigger than even 19's, particularly dark colored ones, look too small.
The 3rd car is 19 inch Te37 G2's and suffers from the same problem, magnified by white body/dark wheels/wheel gap.
Lighter color really helps. Galeforce 370Z with 20 inch TE37 in matte gummetal
Aired up
Aired down
All of this is to say:
18's are too small
aero, stance and wheel color really matter