In general, the right size running shoe is a half to a full size larger than your everyday shoes, depending on the style of the shoe and whether you have any insoles or orthotics to insert. For example, my everyday shoe size varies from 8.5 to 9, but my current running shoe is a 10, in part because I have to accommodate my orthotics. If you're torn between two sizes, choose the larger. Your feet will expand when running long distances (and even short distances). You can always add an insole or wear thicker socks to fine-tune the fit, but you want to be sure to have about a thumb width of space between the end of your big toe and the end of the shoe.