I use 50 gallon tanks for my adult pairs that are specially built for my set up. They measure about 31,5 x 23,5 x 15,5 inch so the snakes have more amount of space in length and width.
For my subadult and hold backs I´ve got some 19 gallon tanks (15,5 x 23,5 x 12 inch)
These are quite normal sizes for keeping in Germany but some keepers use much bigger enclosures.