Running shoes should be comfortably snug around your midfoot and heel, with about a thumb's width of space between your longest toe and the end of the shoe. Generally, your running shoe size is half a size to a size and a half bigger than your dress shoe size. The reason you want this much space is twofold. First of all, when you run your feet will swell. Any shoe that is a little tight will soon become numbing. Secondly, when you run your foot will slide forward a little in your shoe. If your toes hit the end of your shoe repeatedly, the impact will cause you to bruise and eventually lose your toenails. If this happens, your shoes are definitely too small!