If you are talking about tall boots you need to go to a tack store and get measured. It also helps to try on a bunch of different boots because different brands can fit differently. Tall boots are not only sized by the size shoe you wear, but you need to know the height of the boot you'll need (short, med, tall) and the size calf you'll need (slim, full, wide, x-wide). To measure your calf you need to measure the thickest part all the way around. To measure for the height you need to measure from the floor to the back of the knee.
This might help: eBay Guides - How to measure for tall riding boots.