You introduce ammonia and it builds up until you grow enough of the bacteria that eat ammonia to convert it to nitrites. The nitrates have to be removed by emptying part of the water regularly and replacing it with fresh water. Then there's algae, and snails, and fish diseases that become real problems if you don't have good water conditions and minor problems even if you do. Larger tanks mean more stable water conditions, healthier fish, and less sophistication and dedication on your part. Next, you have to decide on a tall or wide aquarium.
Fish generally live either on the bottom, at the top, on in the middle of the aquarium.
Your fish won't be crowded if you have one-third of each type, but keeping all bottom fish in a tall tank is what technically is referred to as stupid. A long aquarium allows fish at each level to spread out, but compresses them together vertically. Some fish are very territorial and spreading them out is a good idea. Then there is the issue of fresh or salt water. If you are just starting out, go fresh.