A 2 foot long tank is fine for a 2 foot long BP, as long as the tank is a foot or more wide (square foot of enclosure per foot of snake is a good rule). As for humidity, what substrate are you using right now? Cypress mulch, or one of those eco earth type substrates hold moisture well and could help quite a bit to get your humidity up.

I personally use a sterilized peat moss/soil mix, and it holds moisture great (in fact, too good if I don't dry it out thoroughly first before putting it in), which keeps my humidity in the 60's if I just spray slightly every now and then or well into the 70's if I give it a good spray. I've never seen anyone speak in favour or against it as a substrate before, but it's worked very well for me so far and being completely organic and pretty soft I can't imagine it could cause any harm if ingested.