When shopping for running shoes, make sure you go to a specialty running store and have the salesperson measure your feet. Even if you think you know your shoe size, your feet can get wider and longer (due to falling arches), even as an adult. Also, it's best to try on new running shoes after a long run or at the end of the day, when your feet are already swollen.
If you're wondering if your current running shoes are the correct size, check the location of your big toe. If it's pushed right up against the front of the shoe, they're too small. Another easy way to check the fit is to remove the shoe's insert and stand on it. If any part of your toes are hanging over the end of the insert, your running shoes are too small.