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.
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..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.