Errrr... well, you could even do a 10 gallon if you want! The bigger the better, just because a smaller tank is hard to keep levels the same. Temp,
ph, salt, everything. A bigger tank will be more stable. 10
gal nanotanks are possible, but I would suggest you try bigger to start out.
A 55
gal is more than okay, especially if you're just starting out. Will be stable, but maybe not as stable as a 100. Go with the biggest that your budget can hold. I had a 29
gal that went okay... but in the end, it's up to your skills and PATIENCE. As long as you don't overstock your tank, you'll be set to go!! Make sure to check out the max. size that your potential fish will get, and stock accordingly. A 55 is good, but might be too small for fish like a yellow tang, or others, that get large. Take a good long while and look through this forum, and check up on some books by Fenner: "The Conscientious Marine Aquarist" .
HTH
~~Rob