There are a lot of species you can look into, so naturally there are some that can do fine in a 55 gallon and then some that can't. My personal belief as there's no enclosure that's too big, there's only enclosures with an inadequate amount of hides. I mean snakes don't get super duper stressed in a rainforest, let's say, because if you've ever looked at one, there's very little open space and a lot of room to conceal oneself. In a large enclosure with two hides, the tides are turned as it's very open- not a whole lot of hiding in between those two hides and plenty of opportunity to be spotted by something with bigger, sharper teeth.
The problem with proportionally large enclosures, when constructed with a large amount of hides, is that it's a lot more difficult to find a snake. I mean they're good at hiding- they've done it for millions of years- so given the opportunity it'd be a whole lot harder to find them in such a large space.
I don't know colubrids, but glimpsing over milk snake care, that should be okay if you provide a few more hides than usual (maybe three or four). You'd be wasting a little bit of height there but that's not a problem from the snake's perspective.
I'm going to add that a really good opportunity to see some of these snakes in person, and acquaint yourself with species you haven't considered yet, is to attend your local reptile show. Repti-con is popular, but if you're like me and they don't come to your state, than the state usually has their own shows that come around. You just have to do some quick Google searches.
Good luck finding some pretty worms for that 55 Gallon- and welcome to the hobby