Use fotowall app. Download the binary file (not the tar.gz) and them r-click Permissions, and make it executable. Then double-click it to run. (https://www.enricoros.com/opensource/fotowall/download/binaries/)
An 8.5x11 page (a4 page, ie 'letter' page) at 300dpi is 2550pixels wide x 3300. (European is just slightly different at 210 x 297 mm.)
In PhotoWall, you can 'Print' the page to a pdf, then view the pdf (change to 100%) to see how it will come out.
To print, you might need to download the driver for the printer. When you plug it in the USB and attempt to print, it will 'Getting Printer Information' and this will tell you the printer name.