Did you mean a 40 gallon breeder? It's tall enough for some climb space [they don't need to climb very high] but there's good amounts of floor space for running.
For a full grown bearded dragon I would get at the least a 40 gallon breeder. It's what we have for our 1 year old and it seems she might out grow it if she continues to grow a full inch every week and a half [yay giant german line!]. You need floor space for them to run but they also like to climb. Ours likes to sit on top of her branches and then climb another branch to the top of her tree house. But then she likes to run back and forth at the very front and then "stalk/hunt" us by pacing back and forth the short side closest to where we sit.