I realize that you want something larger right away. However, this represents a major investment. I recommend you learn on your small tank for a year or so. Find out what your interested in keeping. Once you know that, then select the tank and the related equipment.
Now to try and answer your specific question. There is no one best tank size. This all comes back to what do you want to keep in the tank. Certainly it's nicer to have more water volume but there are other factors too. Consider the shape of the tank. Higher tanks give a big view area, but it makes a much longer reach for working on the tank. Tanks that are wide, front to back, let you do a much better looking aquascape. Tanks that are long work out better for fish like tangs that will swim in the open a lot.
Lastly, the tank is often one of the least expensive items in a SW reef system. Always consider the total cost of ownership, which includes all the filtration, skimming, lighting, and other equipment, plus the cost of electricity, food, and your time and effort.
Good luck.
to RS ! Look forward to following your new build !
