Applications over-ride drivers, that can't be changed.
Luckily, MS Office tends to at least hold onto your last used settings, annoyingly, they use weird terminology.
So when you go into Layout > Page setup > Paper and you see it is on Default tray (Automatically select) you assume that means it will automatically select the right tray, but you would be wrong, it will simply select what ever the default tray is as set on the printer itself. If you try it on Automatically Select it will usually work properly and will print to a printer using whatever page size the document is, and the tray it is loaded in. I have honestly had much better luck with this using the Native Xerox print drivers than I have had with printers that were using the Global Print driver.
But you really have no hope if the document is Letter size and you want it on Legal/Tabloid, something will need to be told exactly how to resize it in MS Office, it doesn't have resize features like Adobe products tend to