In order for the floor of the wedging table to support the weight of the plaster, you will need it to extend to the outer edges of the walls. Measure then cut the plywood to the interior width and depth dimensions of your wedging table plus an extra 4 inches per dimension. (For example, my interior was 289 x 28 inches, so I cut a square of plywood that was 32 x 32 inches.)
If you have excess plywood, so that it cannot lay flat on the bottom of the walls due to the legs, you can either cut off the excess at one side, or you can notch the plywood for the legs, as I have done in this photo.
Notching the plywood allows the plywood to overlap the edges of the walls slightly, which in turn means that the screws will not be near the edge of the plywood once they are set. This, in turn, decreases the likelihood of the plywood cracking between the screw and the edge of the flooring sheet.
Before going further, set the floor in place, without any glue or screws. This dry run ensures that the floor will fit perfectly, without taking the chance of misaligned screw holes or the mess of glue in the wrong places.