If it is a baby, I've found that a thick layer of foam with water underneath and a cave carved from the same foam works great! It keeps the humidity up without allowing the baby to ingest the substrate. You can also cut a little sunken area to create a pool instead of using a bowl, but that really depends on how often you're willing/able to change the water. (Obviously you don't want it to come in contact with the water if it's full of urine.)
As for the tank size, my older female seems pretty happy in a ten for now. She's closer to medium than large though, so I'm sure you could upgrade eventually. I haven't seen her do a lot of moving all at once, but she does seem to occasionally pull out and hop around a bit in the evenings. So I don't think the extra space hurts if you wanna provide it. Though some artificial plant cover would probably be appreciated if you do!