I just prefer a large enclosure. It's more pleasing to me asthetically and the snakes seem to show more interesting behaviors with more space. My corn got to around 5 ft long and I had him for 10 yrs, I think he would have looked really cramped in a 20L. My snakes did always have a very secure and warm place to hide within their enclosure where they could choose to use it or to not use it, so I do agree that a nice safe place is most important.
This is of course just my preference. I never bred any of my snakes though I kept several, so I always found more pleasure watching them crawl around in their enclosure than just sitting in a little box. Our breeder kept most of his snakes in a rack system in smaller enclosures and all were very healthy and well cared for.
Who knows, after you keep snakes for awhile you may develop your own preference and decide to either upgrade your enclosure or keep it the same. Anyone that keeps reptiles ends up deciding that they don't like some of the info they found as beginners and some of it is just fine.
A lot of people would probably think some of my husbandry is outdated, but then again I had snakes for many years that were very healthy so it's all up to personal experience rather than just reading some information.