Sooo you want to put a garter snake in a 6ft cage...(Reddit snake advice)

[Bogertophis]

If you are really into garters I could see doing that, but somehow I don't think that was the kind of idea the person had when making a 6' long, 3' high enclosure. I wouldn't personally waste 3' high on most colubrids, though there are a few arboreal colubrids it would be perfect for.

It would be great for many a larger rat snake- like the Pantherophis rat snake group (black, yellow, grey, etc)- they love to climb branches. Or a Taiwan beauty rat snake, or a Baird's rat snake, etc.