6x16 is enough, although we can always wish we had more. That will get you an outer loop of STD-57 and an inner of STD-42 with enough room for some sidings, buildings, and accessories. If you can somehow stretch your space to seven feet you can get 72" curves, but remember, all of the old stuff (except Boucher) was made to run on 42" curves so the 57" gives you some room to play. 57" should also let you run a Lionel Hiawatha or Vanderbilt if you so wish (they will run on 42, but they will be unhappy).
I say go for it. I find Standard Gauge a lot more fun than tinplate 0. Obviously that's a question of taste (de gustibus non disputandum est) but the big stuff gives me a big smile. I have a loop of STD-87 around the outside of my 0 gauge layout and I love to watch a McKeen car or a 392E and some passenger cars roll. I run mostly scale equipment on the 0 gauge, but for sheer whimsical fun there's nothing like Standard Gauge.