Depths greater than 3 inches do not hurt the effectiveness of the toe kick. Depths less than 3 inches should be avoided.
The optimal height for a toe kick is 3 1/2 inches. Heights up to 4 inches are common. Increasing the height over 3 1/2 inches does not hurt the effectiveness of the toe kick it just takes away from your cabinet space. Heights less than 3 1/2 inches can cause some people problems.