You could go to Carbide Motion and to the Jog tab.
Rapid position to the SW corner and zero all offsets.
Rapid position to the SE corner and see what your X value is.
Rapid position to the NE corner and see what your Y value is.
You now have your basic cutting area dimensions - at the very centre of whatever bit you have in.
Your cutter will extend beyond this though. So for whatever cutter you have divide the cutting diameter by 2 and add it the X and Y values above.
That said - this shouldn’t be a concern unless you’re trying to cut something at the maximum dimensions. If you’re just wanting to cut something 8 3/4" x 9" you can just mount it on the table and zero the machine on your work piece and it will start cutting at that point. Hopefully that helps.