The sizing usually depends on the manufacturer. Most mocassin manufacturers, however, try to make their shoes slightly wider than normal, to allow you to easily slip them on and off. Most manufacturing companies fit true to size. This means that you should fit in your regular shoe size.
Make sure that you do not select your sneaker size. If you want the moccasins to fit more snugly then you may want to get half a size smaller. If you want to wear them without any socks, then you also need to choose half a size smaller than your dress shoes.