I will add to this http://gimpchat.com/viewtopic.php?f=8&t=19476#p265729
The width in image settings is 10.5 inches. It looks like 1.25 " margins are in force somewhere. Go and look in the Epson printer driver settings see if margins are set there.
Also, try ticking that Ignore Page Margins box bottom of the image settings, see what you get. Your 3900 x 5700 is exact, maybe your printer will print margin-less. More often that bottom margin that Ofnuts referred to imposes a new height which in turn imposes a new width. Check that Epson setup for scaling as well, if it is on turn it off.
Edit: Just had a look at the Epson WF-7720 manual. Nice printer but check page 43 for paper size that permits borderless printing and also page 269 for some notes on margins.
This from page 75 check the options again for page size, borderless etc, Wonder what is in More Options tab ?
The width in image settings is 10.5 inches. It looks like 1.25 " margins are in force somewhere. Go and look in the Epson printer driver settings see if margins are set there.
Also, try ticking that Ignore Page Margins box bottom of the image settings, see what you get. Your 3900 x 5700 is exact, maybe your printer will print margin-less. More often that bottom margin that Ofnuts referred to imposes a new height which in turn imposes a new width. Check that Epson setup for scaling as well, if it is on turn it off.
Edit: Just had a look at the Epson WF-7720 manual. Nice printer but check page 43 for paper size that permits borderless printing and also page 269 for some notes on margins.
This from page 75 check the options again for page size, borderless etc, Wonder what is in More Options tab ?