The longer a sanding belt's length, the longer its operational life, since the workload gets distributed over a larger area. You want at least three different types of belts on hand at all times in order to quickly complete each phase of finishing. Grab coarse, medium and fine-grit sanding belts to first remove large debris, to continue without "roughing up" the material and then to achieve a smooth finish. Sandpaper, especially in finer grits, can load up quickly, so make sure you purchase sets of each so you can just replace them when you need to and keep working.
If you haven't yet purchased a belt sander, you need to consider your expected work. If you are going to make smaller pieces, get a stationary sander, which uses longer sanding belts and lets you move the material over the belt. If you plan on making large objects, such as tables, a portable belt sander allows you to sand the piece where it stands.
Your grit material matters. For most applications, you're going to use aluminum oxide sanding belts since they're durable and strong. Once you get into the later stages of sanding, you might want to switch to garnet, which allows you to get a very fine finish. If your piece calls for a very high sheen, consider adding a buffing and polishing wheel to your workshop and using it after your final sanding pass.
Abrasive sanding belts accomplish in seconds what once could take hours to finish. They last longer than regular sanding paper for extra value and convenience. Check the specifications of your sanding machine to know which sanding belt sizes fit and stock up today.