That tub is pretty big for that size snake. I'd say 6 or 15 for sure. If ur hide it too big to fit in a 15 then it's probably too big of a hide. He needs to feel secure! Get him eating a time or two w live and then go for the f/t. Just make sure that u aren't handling him much for a few weeks just leave him alone in a cozy tub and only open it up to feed him. I know it's exciting when u have a new
Pickup but u gotta get this guy eating.

I just had my first hatchlings and one wouldn't eat the first two tries, I set him up w a 6qt, hatchling hide, small water dish...dropped a live hopper in there after about a week and he Didn't Even Hesitate-I also have better luck leaving the hide in the tub while feeding.
Hope this helps