A printing apparatus for printing an image on paper includes first and second units, and a registration unit. The first unit detects a first length, along a predetermined conveyance direction, of paper conveyed from a feed port. The second unit detects a second length, along a direction intersecting with the predetermined conveyance direction, of the paper set in the feed port. The registration unit registers a size corresponding to the first and the second lengths of the paper set in the feed port, based on second unit detection. The first unit detects the first length of first paper conveyed to print a predetermined test pattern before second paper is set in the feed port. The registration unit registers a regular size based on a result of first unit detection on the first paper from among regular sizes corresponding to a result of second unit detection on the second paper.