I say give them as much space as you can afford.
I keep two female garters who are about 2 years old and between 2-3 feet long in a 40 breeder. They’re not big bulky snakes but they are super active and love the space. When you see it stretched out you’ll see that a 10 or 20 gallon is way too small.
I keep mine in a bioactive tank on a mixture of organic topsoil, play sand, spaghnum moss, wood chips, leaves and garden charcoal (NOT BBQ charcoal).
Don’t let anyone scare you away from keeping snakes/reptiles on dirt. It’s great.
Good luck.
