Text Help

One of my professors is super anal about the page layout of our papers, and I am limited in the number of characters to a line and lines to a page. Does anybody know how to limit a text editor to say 70 characters a line and twenty lines per page? I will be using a fixed width font, but I would like to avoid the hassle of fiddling with the margins to make everything fit. I feel like this shouldn't be as hard as it is. I would be happy to hear about ways in Word, LaTeX, TextEdit, or just about anything else.