What size wheels can you use on a mongoose freestyle bike?

If the bike is complete right now, easiest thing is to read on the tire sidewalls. Somewhere there should be embossed the tire size. You can usually go a bit either wider or narrower on the same rims, but you can't change diameter.

If you find them, use the millimeter size. Inch sizes can sometimes be misleading, with several different sizes using the same numbers. There's about five different 26" for isntance which can't be mixed.