The size of tank you will need largely depends on the type of frog you will be housing in it. Smaller anurans do well in smaller sized tanks, but as the number of specimens increases, so does the tank size requirement. As a general rule however, most species do really well in a 20 gallon tank. Many people make the mistake of thinking that if a frog feels dry they'll hop over to the water supply and take a dip. Much as I love the critters, you can't give them the benefit of the doubt when it comes to this type of intelligence.
Some species are very dependant on a humid environment.
This can be achieved in several ways. Keep stale water in it and spray down the tank regularly. Live plants are especially good and retaining the water for some time, though if you have plastic ones it certainly doesn't hurt to spray those too. The other thing to do is to put down moss down moss and keep it wet, wet the soil, or if you don't have soil, you can obtain a moisture retaining product called vermiculite at most garden stores which works really well too. A secure tank top that keeps the tank from drying up, while still allowing for ample ventilation is a really good idea.