Well I must be the odd one....I wont put my adults in anything less than an Iris CB-110 which is about 39"L x 20"W x 6" H.....heck I used to keep them all in four foot Boaphile cages until the collection outgrew the space!
I personally go by the length of the snake. If the snake is 36" than I want the cage at least that long. I house my juvies in 28 quarts.