You normally need your shoe size, the circumference of your calf at its widest point and the length of your leg from the floor to the bend of your knee. I think you measure up the back. That gives your size, width of calf (normally slim, normal, wide, xwide) and height (short, normal, tall).
It is best if someone does this for you.
Also, sometimes half-chaps are more forgiving than boots. My half chaps are medium and fit very well, but there is no way I can get into a medium boot- I need a wide at least.