So much depends on this. If you have rules imposed by room mates, or landlord, you have to take that into account. I'm sure I'm just repeating others since this is page 2 somewhere, but it never hurts to have information reiterated and validated by others when trying to make your decision.
Many would suggest if you are serious about the hobby, start with the tank you want within what you can afford and fit into the space you have. While this isn't a cheap hobby, it is more expensive if you are upgrading to a bigger size a few times before landing on a system you will actually keep up for a while. I personally jumped from a 30 (tank cracked) to 55 to 90 (wanted an upgrade in size and wanted to sump system). On top of the 90 I have a fish only system for fish I don't want to risk in a reef, as well as my old 55 was turned into a freshwater tank for my wife.
Old stuff can be reused and sold, but you will rarely get your money back out of things, especially anything purchased new.
So that is my first suggestion, but what you want, not what is just suggested if it differs.
The next thing to consider is that smaller is not easier with fish tanks. A tank is an artificially recreated ecosystem so until you reach a certain point the system is much more difficult to keep stable. Overall I think the smallest size I would suggest would be a 40 breeder. I think the 20 gallon tanks are also a good beginner option, but you ned to be aware that if you have more than a few small fish it will get difficult to keep up with the waste going into the water.