When choosing the right size aquarium for a fish, there are a few factors to consider, such as the type and number of fish you plan to keep, their adult size, their activity level, and their compatibility. As a general rule of thumb, it's recommended to provide at least 1 gallon of water per inch of fish length to ensure they have enough space to swim and thrive. However, some fish species may require more space or specific tank dimensions, so it's important to research the specific needs of the fish you want to keep before deciding on the aquarium size.