Some people do say that a 20 long is a good size for an adult snake, but for me, it seems too small. A 20 long would be a good transitional cage for me, I think. But there are a few of my adults that would seem rather cramped in there I know.
While I'm one of the abhorrent ones who keep their snakes in plastic tubs, I'm unlike everyone else I think. My adults are in 96qt Sterilite tubs and 41qt Sterilite tubs with a foot print that's roughly 34" x 18". And my adults use every bit of floor space, so I can't imagine keeping them in anything smaller.
Even when I plan on getting display cages for them, ALL are going to be 36" x 24". I just don't feel right keeping adult snakes cramped into as small of a box as I can find just so I can fit more snakes into an alotted area. That's just not how I keep animals.
Each snake is different. If you feel comfortable keeping one in there throughout the entirety of its life, that's your choice to make. But from experience here, none of my adults would be happy in one.
Just remember, one snake per viv. Isn't that right, Broken?