It ultimately will depend on the sex of the snake. If it's a male he could live his entire life comfortably in a 10 gallon. If it's a female, probably the first 2 years it would be fine in a 10, but most female garters get a lot larger and I couldn't see them being too comfortable in a 10. Not that it wouldn't be do-able, but I know my garters well and they use ALL the space you give them! If you think you are going to eventually upgrade with a 20L regardless, I would simply start with that and be done with it. Unless, you get a 10, grow up your snake, and then get that one a 20, and then get a little male for the 10... see how this starts?! it's crazy!!!

As for hideys, you can never have too many places to hide! But a baby would probably feel a little lost in a 20 long. If you want to utilize a 20L for a baby provide 2 or even 3 small water bowls spaced throughout the tank to ensure he finds water. Then you can use paper tubes, pieces of cardboard, wood, rocks, pre-made hide boxes, or even black socks as things to hide in and under. They like to squish themselves into tight spaces, so big, open hides are not ideal. One thing my youngun's like is when I take those woven enclosed bird nests and stuff them with moss. Hit up your local dollar store for some cheap fake plants, they love plants! Another fun one is to take a black sock and staple the opening of it round a cardboard tube, so you get a nice entrance and then a tight space at the back. You would just have to be careful at feeding time, that you don't scare the daylights out of him when you are looking for him because then he probably will refuse food. That's the one major downfall to a large tank for a baby, sometimes they are hard to find and you end up scaring them!