When ordering boots in a general speaking way, they can run small or larger. If they run small then order your regular street shoe size it should be fine. Normally boots are meant to fit snug at first (but not too tight) to be broken in. In normal circumstances, you should order 1/2 size smaller in cowboy boots than your normal street shoe to make up the difference. If it runs small, they generally tell you how much smaller and then adjust accordingly. If they do not state, then your normal street shoe size is practical to order.
If a boot runs extra large, then additional sizing may need to be taken from your normal street shoe size. For example: I normally take a 11 1/2 t o 12 size boot, in Durango sizing I need to get between a 10 and 10 1/2 to make up the difference.