I've used an ordinary 5-gal aquarium tank to make an ordinary live-planted vivarium for 3 BBWTs. Miguelito, Mafalda and Susanita, seeing as how they could well be Argentininian.... And, yes, I can tell them apart)
By all I've read, 5 gallons is not quite enough space, but these guys are thriving. Plus, they tend to cuddle up together. A couple of good hiding spots, (log, oak leaves and sphagnum) and the horizontal branch increase their space options. They DO climb on the branch (and fall off as well).
I've had them for 4 months now and didn't hardly ever see them for the first 2 months. I was feeding them exclusively pinhead crickets, which, upon being dumped into the viv, would hop into the darkest corners, especially into the little log hide where the toads were waiting. So, why ever come out when your food comes to you? I switched to fruit flies primarily, which don't seem to hide. They crawl all over the place. Within the space of a few days, the toads came out on the attack and have stayed out, cocooning under one end of the branch within a nest of sphagnum. They remain on the shy side, but are easy to spot.
Funny thing is that they've only recently discovered the water dish, as I see them with their bum in the water now and again. Fruit flies used to accumulate in the water, but now it's cleaned out every day. They're very avid and efficient hunters.