When you're shopping for running shoes, make sure you go to a specialty running store and the salesperson measures your feet. This may seem like an unnecessary step if your feet stopped growing years ago, but your feet size can change, even as an adult. For example, my feet are longer now than they were as a young adult because my high arches have gradually fallen, due to running. Also, some people have two different size feet, so it's important that you measure both feet. To get a proper fit, it's best to try on new running shoes after you've done a long run, so your feet are already swollen.
If you already have a pair of running shoes and you're wondering if they're the right 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.
Also see: Can I Return Running Shoes?