My 10 year old adult male is four feet long, about five pounds and in a 48 x 24 x 12 pvc cage currently but has spent most of his life in a 36 x 24 x 17 cage. I'll probably move him back to the smaller cage once some of my short tails need the space. He doesn't move around often and mostly stays on the cool end of the cage but I feel there should still be room to stretch out when he does want to. I wouldn't go for a fish tank since heating is so hard to maintain, poorly sized for most snakes and they are a pain to clean but aquarium wise you would be looking for more like a 40 breeder although that's still not as wide as I prefer.