Lessee, first, take two screen grabs of Word running as you would choose to run it (toolbars &c. so that ``chrome'' isn't included in the document area) set to ``fit to page'' one w/ a legal-sized page, the other w/ letter-sized page in landscape mode.
The first graphic will establish the height, the second will establish the width, we used ``fit to page'' so as to eliminate any scroll bars.
Now, open each graphic in an image editing app, if possible, overlay them, w/ the top one set to be slightly transparent, set guides to match the document area.
Now, verify in your Control Panel | Display Properties that your logical screen dpi is set to 96 dpi (the default). Make a note of whatever it is.
Go back to the image editing app, save the file, then re-save w/ a different name. Crop the second version to the document area established above. Now, make a note of the actual image size in pixels, and check the image dpi / resolution setting. If this doesn't match the number from the control panel, set it to do so. Now, check the physical dimensions of the image and make a note of them.
Go back to Word and create a new custom page which matches the image size arrived at above w/ a minimal margin.