Bps are short heavy bodied animals. Like Rich said, length means nothing. The best way to measure a BP is by weight and even then that cannot tell you the sex of the animal. I have an '09 male that is 950 grams and '08 females that are 1,500 grams. They are a year older, but not that much heavier. And he's the same length as two of them.

If you really care you can try to find a breeder in your area and ask them if they will sex your ball python. I know some vets will do it, but I will warn you it's not easy to find a vet that actually knows ball pythons. The reptile 'expert' vet in our area 'sexed' one of my bps by the spurs Needless to say, there aren't any vets in my area that know ball pythons. Your best bet is probably to find a breeder in your area