The thing about boxes in Word is that if you "format" them to be independant of text you can move them anywhere and the text (at any orthogonal angle) goes with them. You can position relative to page and in 1/10s of mm if you ferret around in all "format" sub-pages. Even off the page I think! I uusually leave a faint cutting guide rather than a feature box. You can overlay boxes and make the background transparent and order the layering. AND most important you can lock the boxes together with a "group" command (lower left "draw" tool bar I think).
Bit vague as I am at the library right now.
I am talking about text boxes of course.
If you get American size paper (letter) you would get more width and less waste in the height.
Ther problewm I have with software solutions is that it plays safe and leaves a lot of waste. AND there is still a learning curve.
I got CD cheapo label packs from Maplin and it came with software - **BG** a Photoshop file, a Word file WordPerfect, BMP, etc - and you just printed, ripped-off the saver paper at each end and stuck down an A6 in the position marked. Neat eh?
Crude but ultra efficient - no learning curve. Oh you do have to delete the guide line layers when you print.

SO its for charity? - don't forget the T-Shirts - special paper Matalan T-Shirts at £3 and don't forget to reverse the image before you print! Buy good quality paper and go for white T-Shirts so when the edges go opaque in the wash it doesn't look so bad. I trim as close as poss and design with the trimming in mind. If it says iron for 2 mins - DO. I don't sell these but they would be good enough - wear one first and see how you feel - it sure gets noticed if the logo is big enough!!!