A female would probably need an 8'-10' cage... they can get from 15' to over 20' in length and weigh 200lbs. or more.

And male burms can get over 12'... sometimes up as high as 15' depending.

They are VERY large snakes, male or female.

And sexual dimorphism---females being bigger than males---is common in snakes, so don't give your g/f a hard time. You may end up with a nice welt on your cheek