Are you also making sure the printer driver (computer end) is set up properly with the sizing you want? If the image is set differently in the printer driver than the software, that can cause confusion all around as your telling the printer to read this way and the software is showing it that way.
Maybe try Epson's Print Layout that is free from their site.
https://epson.com/Support/Printers/Professional-Imaging-Printers/SureColor-Series/Epson-SureColor-P800/s/SPT_SCP800SEIt's under the "Utilities" section, but is a fairly easy tool to use and should show you want you will be printing along with the necessary tools on adjusting the way it prints (not color editing like Photoshop or Lightroom).
Also, what OS are you using by chance? If Mac, make sure you're note using AirPrint which will provide a tenth of what the full installation offers. You can't print proper sizes or get all settings like the standard installation would. This is because Apple created AirPrint for mobile device (iPad and iPhone) printing only which limits your options to print. If this is what seems like you have (setting wise), you'll need to uninstall the printer driver and reinstall it, but make sure it's the full install selected.