Like stated above, if you get a young one a 10 gallon will be fine for a while and a pair would even be ok in there...for a while. Actually, a 10 gallon is probably even recommended to make sure that they're getting their food, with a really simple setup(some simple vines and substrate of damp paper towels).
Now, when they get older the suggested space is between 10 gallons per frog, and 20 gallons for the first frog/10 gallons for each extra. They like it very humid.
Also, from what I've heard the young ones are not so forgiving about temperature mistakes. The one I've got is maybe a couple months old(about an inch long) and is doing fine.
Feed them crickets about the space between their eyes, mine is on 1/4 size crickets. Recommended is 3-4 per day when young, I do a little more just to see how much she'll eat. Dust them with calcium once a week.
You may want to think about keeping another frog that is a lot more forgiving first, if you've never kept a frog, just to get some experience. After all, you're probably going to have to go to a breeder to get a RETF. Mainly because you want to find a Captive-Bred one, which are easier to be kept.