The Layout image isn’t a ‘printing’ image - it’s just an image big enough to hold all the pieces. You are restricted to selecting from known paper sizes for the Layout image so that splitting it up into several smaller paper sheets for tiled printing is guaranteed to work. And if you happen to have a large-scale printer or plotter so that it doesn’t need to be split up, then you can select the corresponding paper size and print.
So for most people who print at home: First step - Generate Layout as one big image to hold all pieces. USUALLY YOU DON’T SELECT THE PAPER SIZE IN YOUR PRINTER, but will select whatever larger paper size will work. Second step - Print your one big image as a Tiled PDF to a printer. File/ Layout / Print as Tiled PDF.