Microsoft Word for Mac under layout the only paper option is US letter

OK I fixed the problem and it's all to do with the printer and the driver used.

The printer in question for me is a Canon Laser Printer, model i-sensys MF732Cdw. When you add the printer on your Mac the Mac finds the Canon printer and installs the generic printer driver it has in its files for Canon. This then leads to the default US paper size option in Microsoft Word.

What you have to do is visit Canon's website and download the latest printer drivers for the model in question and then install the drivers onto your Mac.

In system settings under Printers and Scanners remove your Canon Printer from the list.

Then press add printer.

Select your printer from the list.

At the bottom of the screen under a drop down menu called "Use" select this and then choose Select Software.

Type the name of your printer model in the search box and select the correct one.

Then press add and your Mac will load the correct drivers for that printer.

This solves the problem.

When you go back into Microsoft Word all the paper sizes are now available.

As per Microsoft Support Article removing items

Remove Microsoft 365 for Mac applications

1. Open Finder > Applications.
2. Command  +click to select all of the Microsoft 365 for Mac applications.
3. Ctrl+click the application you selected and click Move to Trash.

Remove files from your user Library folder

1. In Finder, press  +Shift+g.
2. In the window that opens, enter ~/Library and then click Go.
3. Open the Containers folder and ctrl+click each of these folders and Move to Trash. Note that some of these folders may not be present.
4. 
   

• • Microsoft Error Reporting
  • Microsoft Excel
  • com.microsoft.netlib.shipassertprocess
  • com.microsoft.Office365ServiceV2
  • Microsoft Outlook
  • Microsoft PowerPoint
  • com.microsoft.RMS-XPCService
  • Microsoft Word
  • Microsoft OneNote

I have the same issue on a MacBook Pro. I have uninstalled Microsoft Word and re installed and same problem