Why does this keep getting asked? Most guys prefer long hair on girls because it's more feminine and fits a girls look more. But some girls can still look nice with short hair. Experiment to see what hair you look best in.
for myself, i like medium hair. no shorter than shoulder length is good. around shoulder length is actually perfect for me. it's so manageable and easy to take care of. i love the look of long hair though. i usually always have long hair, until last summer i cut it shorter and loved it. i'm trying to stick with that now :-)