Below is a list of standard tank sizes. You will need to choose what works best for you, but you also need to consider what type of fish you want to keep as this has a big impact on what type of aquarium you need. The general rule is, the bigger the better. If you get a smaller tank, odds are you will want a bigger tank later. So, go as big as you can afford from the get go, but do your research and make sure you have adequate filtration for your tank size. Decide where you want to tank, decide what you want in the tank, then figure out the biggest tank that fits in that area (and works for your fish) and that you can afford. Last of all, have fun and enjoy!