Most ATV tires are generally sized using the following format: AT25x10-12. The letter "R" may replace the dash to denote radial construction. The first number represents the Overall Diameter (O.D.); the second represents the section width (S.W.); and the third represents the wheel diameter. All measurements are in inches.
Note that these numbers are approximations and individual tire sizes may vary. For example, a 25x10R12 Maxxis Bighorn has the following specifications: O.D. = 25.4", S.W. = 10" (Of course, the rim diameter is 12".)