Altering the Page Layout on the print box alters the Print Area on the map.
The simplest is the Single Page. This can of course be Portrait or Landscape:
Best Fit is used to fit pages of mapping to a route. On the below image, you can see that we are using the OS Road map as the map on screen as it allows us to view a large area. On the map, you can see the route moving from bottom left to top right. We have instructed the print box that we wish to print OS 1:25,000 HD mapping at 100% scale, and with a 10% overlap between map prints. OMN has then worked out the pages required and displays them to you on screen.
Side by Side allows for two pages to be printed covering an area:
To cover a large area, you can use a Tiled layout, such as Tiled 16 Pages: