Need help with shipping box size for a Monday ship

I am seeking opinions on what size box to put my 4' boa in to ship to Texas.
She weight about 6/7 lbs?
I was going to use a 16x16x9. It is a bit big.
The next size i have is a 12x10x7. She fits with 4 pieces of news paper,has some clearnce from the top of box.Not lots of wiggle room,but some.I dont tie the bag tight either.
Shipping price is quite a bit different also.NO i do not want to sacrafice the boa for $30.I just want to be cost effective,and ship her properly.
Thanks
Rich

I would go with the 16x16x9

the size I would use is 15x15x12 (stock size I keep on hand)