If you're going glass aquarium, a 20g long is what you would use for sub adult to adult. I think a lot of people would keep a baby in a 10 gallon until closer to 500 or more grams, though. They can go off food and have other stress related issues if the enclosure is too large.

The kit is a bad idea either way because most of the things the beardie kit comes with are not necessary or helpful for your ball python. Wait for the 1 dollar per gallon sales that both Petco and Petsmart run, as suggested.

Not a lot of people will specifically advocate using a glass aquarium. You should look into some of the PVC cage options which I'm sure someone will link. They are easier to maintain proper conditions and also help the snake feel secure and happy.

GL