Sounds like that'd be great as far as size for your garter. My rule of thumb(but everyone is different) is the snake should be able to stretch full lenght and not touch the sides of the tank, but more is always better....just as long as it doesn't get too big and he ends up getting lost in it. As far as ventilation......good ventilation is very important, and a screen top should be fine. Just make sure it's escape proof. As far as the wood, it should also be fine, but you'd want to treat it with something, since you'll be needing to clean it and such and you don't want the snakes waste soaking into the wood, or any water for that matter. Plain old polyurothane(sp?...it's late
) works well. Two or three coats will completely seal it up, you can also seal the joints in the corners with aquarium sealant. Hope this helps.
Roy