A) The wheel size is embedded into the sidewall of the tire
on the wheel your replacing. (I.E. 27 x 1 1/4 - 26 x 1.75 - 20 x 1.75).
B) The wheel size is the diameter of the wheel
plus the width of the tire. For example, a standard 20" bicycle wheel has an actual diameter of 16.5", but
after you put a 20" x 1.75-2.50" tire on the wheel, the total diameter measures approximatly 20".