The question: "What size tank?" has four parts:
1. How much money do you have to spend?
2. How much time do you have to spend?
3. How big is your house?
4. What do you want to keep?
Point one, unless you have been setting up many tanks for a good long while, you will always underestimate how much it will cost. The solution to this is multiple credit cards, multiple jobs, a second mortgage and lottery winnings. An understanding SO also helps.
Point two, to keep the critters happy, you need to do routine maintenance. If not, slowly but surely your tank will go down hill. If you have plenty of money, see point one, you may consider a maintenance service.
Point three, you need to have a tank that will fit in through the doors. Yes, I know, you can take the doors off. You can also cut a hole in a wall to get a bigger tank inside. For that matter, you can also build an addition around a new tank. There was a fellow who built a 4000 gallon tank in his basement on this board a number of months back. See point one.
Point four, if you want to keep a couple of great white sharks, you will need a serious sized tank
, but if you are happy with a couple of percs, you can get by with a slightly smaller tank.
You have to evaluate your own individual circumstances to see what is "right" for you. Whatever is right for you today may also change later - new job, promotion, lottery winner. Most of us have graduated to a larger tank as we have moved through our life course.
Whatever size tank is right for you, remember to enjoy it.