A:These masks highlight where there is a dramatic change in coloration in an image, serving as an edge detector.
B:The advantages of a small mask is it more precisely indicates where an edge is because fewer elments serve to distort the edge. A larger mask ensures that the highlighted elements are indeed edges by averaging out local changes.
C:If you wanted to detect objects on the horizon a good mask might be [0 0 0; 1 1 1; 0 0 0]