I've asked countless woman at my school and get a lot of mixed opinions and I believe bias as well. I think I can get more accurate results here since girls here don't have to feel pressured with someone asking them face to face. I prefer mostly woman's opinions, but guys opinions are welcome to if they can recall a time when a girl gave them an opinion regarding this matter.
Short hair- from one inch to maybe 3 inches
Long hair from like Enrique iglesias hairstyle to as long as a girls 3 inches plus.
I like short hair better on guys, but I know a few who can pull off the long hair look. It's just that most of the time when guys decide to grow their hair out, they just don't take care of it. It ends up looking ratchet, greasy and messy. Don't like it. If you can't take care of your long hair, it just ends up looking terrible. :P So all in all, long hair only looks good as long as it's being well taken care of (which seems to be a rare thing among guys).
I think short hair is most attractive generally. There's only a few guys I know that can pull off medium to long hair. If you are one of those guys that can look clean and scruffy at the same time, long hair (especially curly or dreaded) I like a lot too. I honestly think it depends on the guy.
It depends on if it suits your face or not. some guys look great with long hair and some guys look better with short. as long as its well kept, I knew this one guy who had long hair but it was dirty, greasy and always looked unbrushes, it was pretty disgusting
Most women like short hair on men. I think it depends on the guy. There's a guy at my school with long blonde hair that looks so sexy. Other than him I haven't met another guy that looks hot with long hair yet.