Longer is great, but girth is King past a certain point. Of course the area of choice to an individual who is keen on "making their mark" would be the other persons heart/mind.
With a longer newsletter, it forces the person to gradually lose interest in the mundane or troublesome details of the readers day, and slowly brings them into the mindset of the author and the information they are trying to impart. But just a long newsletter with rambling sentences can only go so far, the authors "girth" or knowledge base is important as well, for without this a long newsletter is just a one time fling that will quickly become forgotten. A true teacher will wield his words and knowledge together with a passionate flair that also touches the heart/mind of the reader, making the entire process unforgettable and simultaneously instills a desire to seek out further experiences of like kind. Of course when you have length, girth, and a passion that touches another person intimately (heart/mind) then you are automatically unique and a rare commodity, all but guaranteeing return visits for continued rewarding stimulation.