Certificate Widths can be easily extended by Adding following Custom CSS in the WP Admin -> Appearance -> Customize -> Custom CSS
Paste below Code :
.bp-user .mfp-auto-cursor .mfp-content{max-width:800px !important;}
This feature can be used to Print Certificates in Landscape mode. Many browsers like Google chrome provide Landscape option within the print option itself.
Regarding Landscape print option:
Since currently there is not CSS feature to force this layout in all browsers, it is best left for browsers to handle this. However, there are fixes which the user is free to explore. Refer for more information : http://stackoverflow.com/questions/138422/landscape-printing-from-html