Once the letter is typed, go to FILE, select Page Setup, and then go to the tab marked LAYOUT. Choose your option under VERTICAL (top, center, or bottom) and then check "apply to whole page."
I know this seems like a lot of trouble, but you can also do it by your eye...just go to page preview (you should have an icon at the top of the screen)...add spaces to the top or bottom of the page accordingly.
Movinonup 