Marsye,
Put your machine on a table, stand a ruler on end, on the table in front of the highest part of the sewing arm/bed. That is how deep you want your recess or cavity in the sewing-desk/table top.
As far as the size of the cut-out, 12 inches front-to-back by 18-20 inches left-to-right is about average.
But ask the salesperson to check your machine model to make sure it will fit that sewing table.
I was lucky to find a solid wood breakfast table at a furniture warehouse. I called the Dreamworld company who makes cutouts to fit around your machine arm.
http://dreamworld-inc.com/
I used their size recommendations (they were very knowledgeable and helpful) to cut a hole in the table top.
They shipped the cutout while I cut the hole in the table and build a box to hold the machine.
Works great! I will take a picture and post it.