If you are serious about setting up a system, the best thing you can do is start deciding what you want to keep. Some fish and corals require more space than others.
As a general rule though, the bigger the better. You really are only limited by what you are willing to spend. I can't afford more than a 75 gal
setup right now, so that is what I have. People do successful minireefs in tanks as small as 12 gal
. The 75 is my personal favorite though. It's got a nice footprint, and has a lot of surface area to work with, so you can put a lot of different things in it.
Just remember that it's a fair estimate that you will spend about $20 per gallon by the time you are all done buying equipment and livestock. If you get extravogent, you can spend a whole lot more than that!
Take your time and plan well. The quality time you spend planning ahead will more than pay off with more success and better results down the road.