I think 20" wheels/tires are pretty straightforward, you're only limited by what will physically fit on your frame. With 26" tires, its much more complicated. There are 5 or 6 different "standards" for bead size, which is the inner most diameter of the tire where it sits - and seals against - the rim. The variations confused the heck out of me when I first started monkeying around with these things.
As was said, 559 mm is the standard for mountain bikes and for balloon tire bikes. Generally, anything with that bead seat diameter will fit on a set of rims that originally had 2.125" wide balloon tires on it. This includes the 2.3" wide Fat Franks and down to 1.75" wide tires used on some mountain bikes and a lot of non-Schwinn middleweight bikes. That last one is not to be confused with 1-3/4" wide tires. Numerically, it sounds like it should be the same as 1.75", but that designation indicates a larger bead seat diameter that was specific to Schwinn middleweight wheels. While you could slip those tires over your rims, they'd never actually seat on them. Then there are larger bead sizes still, used by lightweight bikes, such as 26x1-3/8". A lot of these are also designated as 650A/B/ or C, and these won't be useful to you either.
Bottom line - as long as it says ISO 559, it's good for the application you asked about. Your only limitation, as with 20" bikes, comes down to how wide of a tire will within your frame/fork.