It is proper Netiquette to tell the the recipient if it is long in the subject line. Do put long in the subject line if it's long.
Long email are not usually read. Write long in the message subject if a message is more than three paragraphs. This mentally prepares the recipient to read an email now or save it for later.
Honesty goes a long way on the internet. If the recipient of an email knows that it is long from the subject line they may read it. As opposed to deleting it immediately when they open it and see its length.
Tell the recipient if the message is long in the subject because it may get deleted otherwise.