Under the Margins tab change Orientation to Landscape.
Click the Paper tab and change height from 21cm to 42cm.
Now click OK.
You may get a warning about the printable area, click Ignore.
You now have an A3 page to work with.
Remember you will need an A3 printer to output the file (if you have one installed on your system you probably won't get the printable area warning, but if you do you may have to heed it as it will affect the finished poster).
If you intend to transfer the file to a machine with access to an A3 printer you should open the Word doc and confirm the print margins are adequate. Margins won't be an issue if the printer is capable of producing larger than A3 prints or has an option to bleed the image, i.e. print up to, or beyond, the papers edge.