This is a little difficult to describe but I think your printer is using A4 paper. If I`m right you can try these tips and let me know. Option One - in your printer settings select A5 paper and match landscape in Word with Landscape printing. If that doesn`t work just change the printer to portrait.
Option two - Keep the printer settings on A4 and portrait. It`s all to do with what the printer thinks the paper size is in it`s cassette.