xiaobautu,pjiang, help me about cover letter.

my attorneay said submitting a cover letter of 16 pages's length is not a good idea because the reviewing officer will not have time to read it thoughtfully and will probably be irritated by it. He also said he had been told in the past by USCIS officials to keep NIW cover letters to about 5-6 pages. But if the letter is too short, it didnot tell things so deaply. Is that true?