It might be easier if you tell us what fish you're interested in. Then, we can help you with how large the fish will get, which lets you know the tank requirements. Usually, if you "google" a specific fish, you're able to look up and see how large it gets and, quite often, the site will tell you the minimum tank size for that particular fish. Was this any help at all?
I understand bettas need little space so that is not a worry. I'd like a zebra snail and 10 neon tetras, they need more room dont they?
I want to have some live plants eventually so need fish that wont eat them all up.
Some Guppys too.
Is this a good mix? How big a tank will I need?
Do I need a spare tank to get the water right before fish go into it?