In another thread I posted pics of twist lock plugs and the four screw mounting variety with a square body is the same as the Smart Plug, interchangeable, according to the installation drawings on their web site.
If you are looking for the physical dimensions of the outside of the power inlet, My Marinco (white plastic, four screw mounting) is 3¼ wide and 3¾ high. The screw hole pattern is also slightly rectangular. The Smart Plug will be virtually the same. All of the square part is on the outside, the dimensions of the flip door are not relevant. Go to my post
HERE to see what the power inlet looks like inside and out.
Charles