welcome back!
they can live off 3in heat tape, but would live more comfortably off a wider band like 6inch heat tape. they would scrunch up closer and shift their coils/body more often over the 3inch.
i'm a subscriber to the growing out the enclosure size as your snake grows philospophy, so i would start with a smaller tub. i personally start out babies in a 6quart tub, but i guess u can use a 20quart if that's all u have. just add clutter. also be prepared to move down in tub size (meaning u have to buy something else smaller than 20quarts) if u start having feeding and stress issues.