When I make sumps I take the size of the display tank into account. The sump should have enough extra space so it will not overflow if the power goes out and the water level in the display drops.
I'm currently building a sump have cut mine so that they are 8 inches tall. I will set some of the bottom of a 12" tank and some towards the top. I'm keeping at least 2 inches between the top baffer and the top of the tank.