Some runners don't realize that they're wearing running shoes that are actually too small for them. Your running shoes should be 1/2 to a full size bigger than your regular shoe size because your feet will swell when you run and you need plenty of room in the toebox. If your toes are crammed in the front of the shoe, you could experience foot problems such as black toenails, blisters, or foot numbness.
When you're shopping for running shoes, make sure the salesperson measures your feet. This may seem like an unnecessary step if you 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.
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.