How much land space?

Do Diamondbacks actually like or use much land space? I have Three mid-sized DB’s in a forty gallon breeders tank. Their land area is a four foot square board that butts up against the outside edge of the tank, so they do not lose any swimming space. But I never really see them use the “full” board. They sit on the edge of the board near the water, but do not venture much beyond water’s edge. So my real quesion would be is building them a cage with a lot of land space be overkill. How much dirt or sand area will they need for breeding. How long, How wide, How deep? Thanks for any help