In my opinion, the optimal "gallery size" that provides professional-grade resolution hinges on the native resolution of the camera (megapixels). The following is based only on personal opinion: 8 MP = 11 x 14" | 6 MP = 8 x 10" | 5 MP = 6 x 8" | 4 MP = 5 x 7" | 3 MP = 5 x 7" | 2 MP = 4 x 6" | 1 MP = 2.5 x 3.5"
Based on the human eye, the advantage gained from greater than 8 megapixels is highly elusive ;-)

Roger-in-FL