When choosing the size of gallon pot, you must think about the final size of your plant. Bigger plants will need bigger containers, while smaller plants grow best in a relatively small container. You need to match the size of your plant with the size of your container.
A general guide is to have up to 2 gallons per 12" of height. This isn't perfect, since plants often grow differently, and some plants are short and wide instead of tall, but this is a good rule of thumb.