When you ask , the actual size is 4” x 6” and the pixel size is based on the printing resolution. The rule of thumb is that 4x6 at 300 DPI is done at 1200x1800 pixels for quality print, according to Visuals Clipping. Having adequate dimensions for the size of the pixels will ensure that any images will be very sharp and maintain detail when printed. Specific requirements may differ depending on the printer and the quality of the printout produced.