Small adult males can do fine in 36x18x18 which is about the size of a 41qt sterlite. I know breeders who keep their females in 48x24x12s and I recenlty saw an 8 ft Guyana male in an enclosure that size. It sounds small but he had plenty of room.

As for the little guy in the pic, a 40 gallon tank will be fine for probably another year and you might as well use it if you already have it. I have a 2007 female and she's still in a 41qt sterlite with plenty of room. She is on the small side but I'm just sayin.