A 20L should be fine for an adult, and fine for babies (separately) if you provide lots of vines and hides and aspen for burrowing. They like vines suction-cupped to the walls and back, especially. I have three adult snakes in 20Ls, one Cali king and two corns. The corns basically do nothing but sit in their hides and stare out. When they are feeling hungry, they might come out and hope to be fed. They certainly aren't pacing around the viv wishing they had more room. The kingsnake is active and would probably make use of a larger viv, although she isn't a large snake, just in the 400g range.