Printers usually can't print to the edge of the page so I have to add a margin around each page.
You can adjust the margin width using the option near the bottom of the web page where you enter the design's size. If you set the margin too small for your printer, your design may crop or shrink.
If your printer supports borderless printing, you can set the margin to 0 and turn on borderless printing in your printer's properties/settings.
If you want your design to fit most of a page without getting into the margin/borderless issue, enter (for letter sized paper) 7.5 x 10 inches as the size.