To describe someone who has long hair one could say:
トムさんのかみはながいです。 Tom's hair is long.
However, it is more natural in Japanese to say:
トムさんはかみがながいです。 Tom has long hair (or As for Tom, he has long hair.)
This not only applies to hair length, but to most descriptions of a person's physical attributes.
A さん は [physical attribute] が [description of attribute]