There are lots of very useful "sticky" threads on this section and on the "BP husbandry" section. Those would be the first place to start.

A 20-gallon tank can be totally fine for a baby, as long as it has enough clutter in it - an assortment of hides, boxes, tubes, plastic plants, etc. What they really hate is feeling exposed and out in the open; they need to be able to get from one cozy hole to another to thermoregulate, without having to cross an empty scary wide open expanse to do it. In a glass tank, you'll also need to cover over most of the screen top in order to retain humidity, and you might have to wrap it with insulation around three sides to reduce heat loss, depending on the room temperature. And the snake will outgrow it eventually.

If you already have a tank and want to set up an enclosure you can see into that looks nice, a 20-gallon tank can work perfectly well. But if you don't already have one, it probably makes more sense to use a tub, since they're cheaper and require less modification, and the snake will outgrow it anyway. You could just get whatever enclosure you think you'll use when the snake grows up, but then you'd need to fill it up with even more assorted stuff, which you would then have to clean, etc. Baby-sized hides don't really take up much space.