I kind of like this breakdown..I have 4 categories of coral size. 1 is a booger, which is either 1 polyp or a piece less than an inch, 2 is a frag, with 2-5 polyps or a 1-2 inch piece, 3 is a chunk, 6-10 polyps or 2-4 inches, and finally we have a colony which is either bigger than 4 inches or has 10+ polyps. So I would say that a euphyllia colony needs to have 10 polyps.