It's all subjective and dependant on wheel design. For instance this BBS would not look as good in 18" size:
All has to do with number of spokes and their shape, generally the thinner the spokes, the better it will look at 17"...
And the wheels I have on my FR-S, I prefer in 17" over 18". Saw someone with 18" version at a car meet and it just didn't look right to me:
17"
18"
