Chisos boots fit true to size according to US shoe standards. Simply order the size that you wear most often.
For men, generally boots will match the same size you wear in dress shoes. For ladies, a half size smaller than your athletic shoes. The conversion between men's and ladies' is approximately 1.5-2 size increments. For example, a Men's 7 D is a Ladies' 8.5 or 9.
Exchanges are free in case you want to try a different size.