Hi Michelle!
It is based on your normal walking or running stance. My treadmill at home has the normal width, I think 14 inches, can't remember. I have never had any problems running, and I use it pretty often. However, DH has to go to the gym to use a treadmill with a wider belt due to his normal walk/run stance being wider. His feet would keep touching the edge of the belt and the side of the treadmill due to his cadence. He is also flat footed so that might make a difference. You can always go to a sporting goods store and try running on a few and see how wide the belt needs to be for your comfort. You should be able to tell after a minute or so if you feet remain comfortably inside the belt width without touching the edges of the treadmill. Good luck getting a treadmill from Santa.
Kristi