If printing from LR, first go to Page Set-Up (Print Module, lower left side button), and for Media Size, select the size and the feed source from the drop-down menu available to the side of the size indicator.
Then go to the next button, Print Settings, and make sure to select the correct Media Type relative to your profile, and any other settings you may wish to make. Then click on Presets, Click on Save Current Settings as Preset and give it a name. This will preserve all the settings in Print Settings, but not the choice in Page Set-up. This choice will remain sticky unless you change it, and should always retain the selected Media Size unless you change it, whereupon the new one will be sticky until the next change. Then load the paper by inserting it through the FFA grey tray feed, when it is properly lined up, press "Load" on the LCD and you should be good to go.