I just tried doing this in Word, and it will work if you do the letter as WordArt instead of just typing in the letter:
1) Set up your page with whatever margins
2) Click Insert and then from that menu bar, click on WordArt. A menu of styles will pop up.
3) Pick the 1st one - looks like outlined letters with no curves or other effects
4) Pick your font and type your letter. Don't worry that the available font sizes don't go very high.
5) Word will kick you back to your document, with the letter in a box. Now you can use the "Shape Fill" option from the menu to fill the shape solid black, or just leave it outlined (not sure which you'll prefer for what you're doing).
6) Now grab one of the corner sizing handles on the box and drag it out so it fills the whole page.
I used Word 7 to figure this out, so depending on your version, it might be a little different, but it shouldn't be too terribly awful to figure out. Let me know if any of this needs clarification or if you want screen shots.
~~Cheryl, with a "C"