I have a HP Deskjet All-In-One 2540 series that is compatible with the iMac I can print, scan, and use AirPrint. After selecting my picture in photo, under help I typed preview which gave me the "Edit with Preview. Under paper options I had to select A6 (paper size 4.13 by 5.83) and selected (Print Entire Image). My complete picture printed. The option to set up "Custom Paper Size" did not work to set up 4 by 6 paper size but I can print pictures now by hovering over the size of the paper options and selecting the one closest to my photo paper size.
Thank you so much for you response and the picture. You helped me to put my thinking cap on and solve the issue.