I would stick to a 20g long. This is of course just my thoughts, but a 40g breeder is huge for just one frog even if he is a male. In my experience keeping different reptiles and amphibians over the years, animals can easily be stressed out with a too large of an enclosure. I have a 40g breeder, but i have 3 adult males in it and surprising they are very happy when some might consider it a bit over crowded. If you were to get a 20g long and split it 60/40 with water being the 40%, he would do just fine. Going off of the care sheet listed here, they say a minimal 15 Gallon enclosure for an adult male, anything over that is plus, but a 40g breeder is a large tank to fit in your home as well. I know mine takes up a large area in my living room.