The size of the tire will do.......20", 24", 26", 29er. The larger diameter will attain a higher speed than that of a smaller diameter. A smaller diameter will have faster acceleration than the larger.
The way I do it is to put a piece of tape on the rim, and on the ground next to the rim. Them roll the wheel one revolution, and put another piece of tape on the ground next to the rim. Now measure the distance between the 2 pieces of tape on the ground. Divide this number by 3 to get the diameter.
There can be quite a bit of difference between a 2.125, and a 1.95.