A 55 Gallon aquarium will make a fine terrarium display. I am set up more for breeding now,so am using much smaller enclosures. When i first started with dart frogs my first display was a 55 g. I had a natural cork backround,gravel bottom,abg soil ( with fiberglass screen keeping soil and gravel seperate ). I kept a layer of spaghnum moss on top of gravel- then live tropical moss (sometimes available from Black Jungle ) on top of that. I had a seperate area on one end of aquarium with a 4 inch high piece of glass siliconed in so i could make a waterfall area without worrying about soil mixing in with water. With good lighting you can grow broms mounted on cork or add some mopani or malaysian wood and mount broms on that. Plant some climbing type plants to grow up cork backround ( ficus works well ). Also there are lots of other plants available from vendors on dendroboard that do great. Good Luck !
the bigger the tank the more you can do with it and the more frogs but you may not see the frogs as often as you would in a smaller viv im gonna be building a 150 gallon somtime this fall to. Honestly allot of people use those size tanks for display so its not really unusual
I just want to say as a quick note, if you want to go out and buy a 55 gallon I would say go with a 40 gallon instead. A 55 has some akward dimensions (tall and narrow, not really the best it could be for either terrestrial or arboreal) while a 40 breeder, in my opinion, offers great options for landscaping. In addition they make seem to make amazing verts (afaik)...
I have a 55, it seemed like the perfect size to be able to say you have a big tank. Its got just the right amount of ground space, and even more wall space to throw any plants you want in it. The only thing I could throw in as a con is that majority of 55g tanks are tempered glass... So no drilling practice!