All depends on spoke pattern.
I run 18's that many people confuse with 17's because of the spoke pattern. (Deep dish wheels with shorter spokes)
If you get 17's that you want to look larger than they are, get a spoke pattern that goes to the edge of the wheel.
For example, both 17", but one looks bigger than the other because of the spoke pattern:
Spokes that go to the edge on 18" wheels look HUGE! Just imagine the wheels in the above picture are now 18".
Here we have OEM 17's vs my 18's:
OEM 17's vs after market 17's that have a much smaller spoke pattern that don't go to the edge of the rim: