If you are like me, you want the top to overhang a bit, so the back does not show on the front side.
When I layer my sandwich (before birthing), I pull the back a scootch tighter, about 3/16-1/4" shorter than the front, easing it all around the outer edges. When I birth, the back is smaller, and pulls to the back, thus disappearing.
This makes the front a tiny bit bigger, so you can get puckering in the birth opening. Try to have no ease at the opening, just on the sewn sections.
When you sew around the sandwich (before birthing), you want to make a 45 degree turn and sew off the edge of the seam allowance. (The colons represent your seam line to the outer edge of fabric.) I cant send a pic or a EQ drawing so try this:
-----------stitching line---------------: (Opening) :-------------------
________cut edge______________: (opening) :______________
When you turn it inside out, these sew-off-the-edge bits pull the seam allowance inside the placemat.
Also, if you use some kind of "batting", that helps keep the placemat from puckering. I use flannel. Gives the placemat another layer to protect the wood from the heat of the dishes, and a bit of absorption of liquids.