When the wheel size gets any bigger than about half of the hood height or 1/3 of the roof height they overwhelm the rest of the car visually, and distract the eye from seeing the car as a single item. You see two large wheels, and oh, by the way, that's a Mustang in between them.
The flip side is also true (less than about 25% of roof height or 3/8 of hood height looks lost).
20s should fit ive seen a few of them with 20s. if it where my car i would go with 20s but its up to you alot of people are gonna say u are dumb for puting 20s but its your car, what i can tell you about 20s is to be carefull when you drive cuz they are delicate especially if you have a low pro tire telling you from experience.
I have seen a couple of Stangs with 20s that I didn't mind, but generally, the 18s look much better to me. I agree that it is a proportional thing. I think the 20s on some SUVs look fine, but 22 or 24 look dumb. From a performance perspective, the 18 win. Style is a little more subjective.
Printable Version
18's or 20's? - 
