Smaller sheets get lost in filing and larger sheets must be folded before they are filed. Make your correspondence the best quality output your machinery can produce. If you use a typewriter, use a fresh, black ribbon. Dot matrix computer printers are acceptable only if you use a quality ribbon and change it often. Paper should be the best grade you can afford.
Genealogical correspondence has a way of staying around for a long time.
For your handwritten materials, use black ink because it reproduces well. Always make copies of the letters you write. Even with a handwritten letter, you can slip a piece of carbon paper beneath your paper. With a computer or word processor, it is easy to produce two copies, one to mail, one for your files. Print out copies of your e-mail messages and replies that need a permanent place in your files.