Treadmills come in different belt widths. From a workout perspective, what is the importance of belt width?
The narrower and shorter the belt is, the less suitable the treadmill will be for higher level workouts and taller exercisers.
At the minimum, a 16" wide x 42" long belt is going to feel very cramped, and will not really be good for running, as even an average height individual will need a longer belt to accommodate strides. 18-20" is preferable, and if you are going to be running at any type of speed, 20 or 22" is better, with the attendant increase in the length.
It also isn't so much the belt width by itself, but what that means for the rest of the construction as well. A wider belt is (generally) going to have a higher HP motor, which means that it can reach a greater range of speeds, and will not struggle to keep up with bigger runners (Size or weight). By the same token, the deck underneath the belt will also be more likely to be stronger, and either cushioned or have other compensation. Some treadmills will also have energy return, this may or may not be important to you.
So if you are not really tall or heavy, and don't intend on running at high speed, then a narrow, short treadmill may be just fine. If you are taller, heavier, or intend on running at increasing speeds, then you need something that will accommodate the longer stride.
One last consideration - If your body morphology is such that you have a non standard gait (extremely splay footed, bow legged, whatever), your foot strike pattern may also dictate that you need a wider belt. If you can't try the treadmill, then try running in dirt where you can look at your foot strike pattern, and see if there is a minimum distance (outside edge to outside edge of the footprints) that you might need.