Standard sizes

http://zazzle.custhelp.com/app/answers/detail/a_id/181/~/poster-and-print-sizes-explained

Standard Print Sizes/Dimensions (and minimum resolution requirements)

 4" x 6" (600 pixels x 900 pixels)
 5" x 7" (750 pixels x 1050 pixels)
 6" x 6" (900 pixels x 900 pixels)
8" x 10" (1200 pixels x 1500 pixels)
 8.5" x 11" (1275 pixels x 1650 pixels)
11" x 14" (1650 pixels x 2100 pixels)
12" x 12" (1800 pixels x 1800 pixels)
12" x 16" (1800 pixels x 2400 pixels)
 16" x 20" (2400 pixels x 3000 pixels)
18" x 24" (2700 pixels x 3600 pixels)
19" x 13" (2850 pixels x 1950 pixels)
20" x 24" (3000 pixels x 3600 pixels)
24" x 24" (3600 pixels x 3600 pixels)
12" x 36" (1800 pixels x 5400 pixels)
24" x 36" (3600 pixels x 5400 pixels)