It is important that you take into consideration the size of the running belt when purchasing a treadmill. And by size, we mean not only the length but the width of the running belt as well.
A rule of thumb is that the faster you run, the longer and wider your strides are.
Of course, the taller you are, the longer the running belt needs to be (as your strides will naturally be longer too)
Finally, when the right belt width, it is important to know how comfortable you are with running on a treadmill. If you're just starting out, or naturally have a wide stance, a wider running belt would be more comfortable for you.
This way, you do not have to worry about accidentally stepping on the frame of the treadmill in the middle of your cardio workout.