One good thing to find out is what size frame it is, not just the wheel size. So generally speaking, the frame on a bike with a 26" wheel set is going to be bigger than one for a 24", but that 26" frame might come in a small, medium, or large version. I'm exactly 6 feet tall and if I ride a small or medium size frame on a 26" bike, my toes have a tendency to brush the front tire on turns. Then again having size 13 feet might have something to do with that too.
If the seller doesn't know what size frame it is, or you can't find a "S", "M", or "L" stamped on it somewhere, the length of the seat tube will tell you if it's the right size or not. If you're around 6 feet tall you'll want it to be somewhere around 19 inches. You can still ride one with a 17" seat tube, but it'll get a little cramped for long rides.