What size BMX wheels?
 

A long time ago I found this deserted BMX bike with no wheels and some other trashed parts. In the past I tried putting on old wheels I had laying around but every time I ride the wheels pop out of place (or at least the rear one does). How do I figure out what wheels to get?

When I originally found the bike I was a kid with no money, and now I'm much older and realize I still have this old frame laying around and would like to do something with it. I think I'm going to turn it into a cheap school bike for getting around at my University campus. Light weight and less likely to be stolen.

The overwhelming majority of BMX bikes use 20" wheels.

What do you mean? does the wheel go sideways in the frame? In that case, look for something called a chain tensioner to help keep the axle in place.

The only other thing I can think of would be badly mangled dropouts, and fixing that would be an entirely different project.