Hello, I am writing a business letter and my name and the enlcosure line is running onto the second page. What is the rule of thumb for this situtation?
I think you should do whatever you have to to get it to fit on one page. You could try using a different font, or using a smaller size. You could try expanding the side margins or the top and bottom margins. What about adjusting the line spacing? A long letter should be single spaced anyway. Best of all, go back and delete stuff. I'd rather omit the "enc" line completely than let a letter run into two pages.
If you absolutely can't make it fit onto one page, then you'll have to make it look longer so that there's at least one paragraph on the second page to go with the signature and enc line.
You can do the reverse of the previous suggestions: Make the margins deeper, make the font larger, find a bigger font. You might be able to adjust line spacing (from 1.0 to 1.5 in Word, for example.) You might also be able to drop the body into page 2 by adding space between paragraphs -- there's usually a command for that in Word.
Readers generally do not notice (consciously anyway) details like margin size or line spacing. At the most they may get a general feeling of "nice to read" or "unpleasant to read" or "easy/difficult to read." That means that you have a lot of leeway in tweaking the details to end up with the effect you want.