An 18x7.5" wheel with at least a +42mm offset would fit a civic. With no lowering, I don't think there would be a problem with rubbing. I would just advise against it because of the fact that there isn't much of a sidewall (tire size would have to be 215/35/18) protecting the wheel against potholes and bumps and whatnot (especially if you were thinking of 18's as a daily driver wheel)

Also, 18" wheels would look kinda off on a car that isn't lowered or has no body kit (or both).