There is a direct relationship between image resolution (and so the file size) and photo quality when printed.
For your reference, below, you can see what we consider the minimum required resolution for printing your photographs.
2.5" x 3.5" (6.4 cm x 9 cm) - 240 x 320 pixels
4" x 6" (10.2 cm x 15.2 cm) - 480 x 640 pixels
5" x 7" (12.7 cm x 17.8 cm) - 768 x 1,024 pixels
8" x 10" (20.3 cm x 25.4 cm) - 1,024 x 1,536 pixels
16" x 20" (40.6 cm x 50.8 cm) - 1,200 x 1,600 pixels