Are you talking about the size of the app on the Desktop or the size of the Word page when MS Word is open?
If you're referring to the size of the Word page when MS Word is open, go to the Zoom control at the bottom-right corner of every Word doc which controls the size of the page. Don't use the "Maximize"1" icon in the upper-right corner of your window to enlarge your doc. Instead, when one of your windows opens and is too small, try changing the Zoom number to the size you want, and close Word. When you re-open Word your word page should be the size you Zoomed to.
If this doesn't work, you can open the Ribbon, go to Page Layout, and then the Page Setup section. Click on Page Size and follow the on screen directions. Then go into the View section, select Zoom, and set your Zoom size.
If you were talking about your Desktop, you can manually placing your cursor on each edge of the window, and dragging that edge out to the edge of the screen --- thereby manually maximizing the page. (Again, don't use the Maximum Size 1 button in the upper-right side of the toolbar.) Then reboot. You may have to do this a few times, but Word should remember the new page size after one or two reboots, and open all pages to that size every time.