What most of us use is the D8C which is a 3x8 like the nortons. Using a shorter plate might work but IMO you're better off with the 8" plate. Going above 325, to the 600, or whatever is not recommended by Dia-Sharp, the maker of DMT. The plates will be ruined in the higher grits if used to lap waterstones. When using the D8C 325 they recommend doing so under running water. This will clear the swarf and prolong the life of the plate.

For a new DMT take a plane iron, chisel or if , like me, you haven't got those, a long heavy screw driver shank rubbed on the surface of the plate, paying special attention to the edges, will take down any high spots, divots that might scratch your stones. Do this with water with a couple of drops of dish soap in it. If you sharpen your kitchen cutlery on the plate the water/dish soap is also recommended by DMT.