The general rule of thumb with ball pythons is that floorspace should be about 1 foot greater than the length of the snake. That means that an 18" little ball can happily live in just about 2.5 sq ft of floorspace, give or take, and a fully grown female at 5' long should have around 6' of floorspace, so a 3'x2' or similar dimensions will be perfectly adequate for a big female.