The specs say the Epson WF-7720 prints up to 13" x 19"
The Windows Gimp print plugin is just a link to the Windows printer driver supplied by Epson
First look in `File -> Print -> Preferences` which should take you to the Epson Printer Driver. Check that there is a Super B sheet size, or maybe a User Defined Setup 13" x 9" . (certainly not possible on my lowly Brother printer ) . That might let you set up margins and ppi in the Image Setup.
- winprint.jpg (111.94 KiB) Viewed 1445 times
..or..maybe not There have been reports that Gimp always defaults to US letter size.
The Windows Print is known to be less than wonderful, after all Gimp is a linux application.
Use some other program for printing. Did the printer come with any image editing application ? Even a cut-down one but known to work with your printer. Export the image from Gimp and use that for printing.