Chisos boots fit true to size according to US shoe standards. Simply order the size that you wear most often.
Generally boots will match the same size you wear in other leather footwear, or a half size smaller than your athletic shoes. If you normally waiver between two sizes, select the smaller size.
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.