Disclaimer: every use of "you" and "your" in the following response aren't directed at you in particular - they're general statements more directed at myself than anyone else.
My thought: The smaller you make your
head look, the bigger your body will look in proportion. So long hair is actually not great because the weight pulls it flat against your head, and really short hair isn't great because it doesn't add any volume to your head.
Probably the best option is to go chin length, and get it permed or something so it's got some volume. Not the way I'd go personally (because it sounds like way too much upkeep), but in my mind that's the way to go. But really it's not about hiding your weight - that doesn't work at all. It's about having a flattering hairstyle that doesn't accentuate your weight.
I also have long hair and although I'm not very emotionally attached to my hair (every so often I hack it off to chin/shoulder length or so, but right now it's to my lower back, which is long given that I'm 5'10 and have short legs... but I digress), I've put a lot of thought into what kind of hairstyle looks best, and in particular, what won't make me look even fatter.