It really depends. I would as stated before, recommend a 55 gallon plus. If you really really want to start smaller I'd probably recommend an aquapod or something of the sort ( an all in one smaller tank). But, from my experience as being sort of new into the hobby my 55 gallon is MUCH more stable than my 10 gallon QT tank. The water temp changes/ swings daily in the QT as well as the water evaporates much more quickly in the 10 gal
as where the 55 gallon does not fluctuate at all ( water also evaporates more slowly). If you ignore a 20 gallon for 1 day you'll have problems, if you ignore a 55 gallon for 1 day, not too many issues usually.
I got my 55 gallon relatively cheap. If you have a good place to put it and dont need a stand you can sometimes find 55 gallon 'kits' for $150 or so. My whole set up before adding equipment was about $300. Its possible! Keep in mind too, you dont have to buy everything at once, as it takes 4-6 + weeks to cycle the aquarium, giving you time to save up!
I'd also recommend starting with a FOWLR
( fish with live rock ) tank to save some money on lighting at first when you start up. Not only is the lighting kind of pricey, but SW
critters ( especially coral) are hard to keep for a beginner in SW
. Ive had my tank for about 6 months now and probably wont start corals for a while yet!