Questions about the Canon G1 X's AF Flexi Zone and maximizing DOF

If I want to maximize DOF for a landscape shot (aperture set to f/8), will moving the small focus box to about 1/3 down the middle from the center of the zone do the trick? If not, will I achieve DOF by keeping the small focus box at the dead center of the screen?

What's the difference between setting the focus box to small and normal (default size)?