I started him off in a 10 gallon but he was scrabbling at the glass all the time. In the 20 gallon, he seems a lot happier. & I'll go larger for him, when we move!
If you're going to bring a toad in from outside, you probably can't go 'too large' for floor space, so if you've got the room, a bigger tank allows you to have more fun planting a terrarium anyway, if you want to go that route!
They do like low, cozy hidey-spots, so of you put in some cork bark or similar, he'll prolly hollow out a nice den underneath it; ditto for ivy or whatever else you may plant. (it always seems to me that those 'toad houses' you see in garden catalogs are too tall, at least according to what the toads with whom I'm acquainted prefer...really, I don't think they care for cathedral ceilings! 
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~kimhotep 