This is a layout offering 1 hold for every 2 square feet which is industry standard for Middle School aged children up to adults.
You'll notice the small green holds are concentrated at the bottom of the wall. Small holds tend to be for feet plus they are cost effective. You normally do not need the blue medium holds as foot holds but they are nice when your climbers need a little help taking more weight off of their hands.
Tip: The larger the foot hold, the easier the hand holds will feel.