15" wheels will def look lost on a s13 or s14.

Typically the safe wheel combo for an s13 is (Front 17" or 18"X9" +20) (Rear 17" or 18"X10 +20) A fender roll and you are set. Tire size is important as well. 9" 225/40 and 10" 245/40. A stretch will allow you to have little or no problems.

Bottom line don't go with a 15".