With Letters, Pay Close Attention to Paragraph Depth
My two cents is to also pay particular attention to paragraph depth and let that guide you on how long the letter should be, especially with prospecting. Remember that readers "scan" letters, they don't start at the salutation and continue through to the postscript. If your letter is filled with dense paragraphs, your readers will have difficulty scanning and picking up the benefits and kernels of information that will interest them, keep them involved with the package and then hopefully see the desired action taken.
It's better to have a multi-page letter where most of the paragraphs are one, two and three lines deep than a one-page letter where they are five, six and seven lines deep. Yes, the old rule that "no paragraph should be more than seven lines deep" is true. But I'd add that one that deep should be found in letters very occasionally.
John Bethel, Copywriter, Richmond, VA