The rim diameter determines what size tires will fit. Terms like 700c, 26 inch, and 29 inch are often used to refer to wheel sizes. But they are misleading. None of them are accurate, because they are an approximation of the diameter of the wheel INCLUDING THE TIRE. That is a bad way to specify the rim size because the size changes just because you put a bigger tire on it. The ISO method of specifying rim sizes is based on the diameter of the rim at the point where the bead of the tire touches the rim. This is called the "bead set diameter" (BSD). A so called 700c is actually 622mm diameter. A so called 29 inch wheel is also 622mm. A BSD 622 tire will fit the rim on a hybrid bike, and also on a 29er"
The "bead" is the metal hoop part of the tire that presses against the rim when the tire is inflated. Note that this diameter is not the outer diameter of the tire. The terms 700c and 29ers are terms that refer to the outer diameter, not the BSD. What we used to call 700C is actually 622mm BSD. There are three sizes of wheels currently in the stores:
Nominal Bead Set Common Use --------------------------------
16" 305 mm folding bikes with small wheels, eg: Brompton
20" 406 mm folding bikes with large wheels.
26" 559 mm mountain bikes up to a couple of years ago
27.5 584 mm some mountain bikes, wierd size
700C 622 mm all road bikes, hybrids, and 29er mtn bikes
29 inch 622 mm mountain bikes
From the document: Help for Wheel Lister