Tell your audience what you can offer, the reason for your existence, how you can help ease their life!

I also agree. Try to be personable and just say what you wish in order to connect with your audience. It can be whatever length you feel is best.

It depends on how much you have to say about yourself. Some are short, whereas others provide loads of information. It depends on the type of site you have, and how integral you are to what it is you're offering.