As tiny as she is you can go with a 10, but make sure you have two nice tight hides so she'll feel secure.

Mesh lids SUCK for keeping temps and humidity correct. If they have mosty solid ones instead get one, otherwise you can use plexiglas or foil to cover most of the lid to help stablize things. Also, don't think for one second that your new snake won't be able to escape if it wants to. They are solid muscle, and can exert a great deal of force against something they want moved. Use latches or weights or even straps to hold the lid securely.

The Accurite thermometer/hygrometer you can get from wal-Mart real cheap is all you need for measuring temps and humidity. Make sure you get a thermostat to regulate the UTH.

A young BP should be happy in a 10 gallon for the better part of a year before you need to upgrade.

After you get your new baby, give it a week to get used to the new place before you handle or feed. The snake will be stressed out and need some quiet time to get used to things. Then go ahead and feed as usual.

Study the care sheets here, they are an invaluable source of info.

And welcome to the addiction!

Gale