Like some of the other questions of the week, the only really correct answer is "It depends...". What might be right for one person would be totally inappropriate for someone else. Choosing a tank size is a bit of a balancing act in that you need to consider many important factors.
I would consider the most important factor being, what do you want to keep in the tank? For example, if you want to keep dwarf seahorses the tank sizes mentioned previously. of 90 gal, 120 gal, and 240 gal, would be far too large, because of the feeding requirements. On the other hand, if you wanted to keep a small shark species, all of those tanks would be too small.
Also important is you budget. The bigger the tank, them more it's going to cost you to set up. No point in trying to set up a tank that you can't possible afford. While we are on the subject, you must also consider the cost involved with running the tank. Electricity, salt, food, test kits and so on are all ongoing expenses. Your also going to need to replace equipment from time to time. Long term salt water will damage just about anything. Over time, you will likely need to replace lighting, pumps, heaters, filters and filter media.
Then you need to consider your space and lifestyle. If you only have 3 feet of space, there is no way your going to fit a 4 four tank in there. If plan to move in a few years, it's a lot easier to move a 30 gal tank than a 90 gal tank.
Personally I tend to like larger tanks. However, some people have created some real gems in small tanks. It depends...