The size Is fine for a hatching to start with, should last a few months but he would need to be upgraded when he is bigger (which won't take too long).

Carpets are a lot more active then bps so they need more room to move. Having only the foot height is ok if that is all you can manage but when he is bigger you should try to get him more height. Jungles will still climb some when adults so they will utilize the height. Mine spends a lot of time on his perch.
Coastal carpets on the other hand climb a lot less or sometimes not at all when adults.

A carpet is a great choice. You'll enjoy it. Going from a bp to a carpet is kinda like going from a rock to a snake.

Some carpets can be nippy as hatchlings. If they are just keep handling him and they tend to calm down over time. A bite from them when small doesn't hurt at all.