The size of your table top. (Expensive table pads are made to fit the top.)

ML

I've cut mine a bit smaller than the tablecloth size. The table pad is soft enough and I figured it would add just that much more protection to the apron of the table.

Having studied it a bit more, I think I'd like an elasticized one. That would soften the edges in a way that I would like, and ensure no sliding around.

There was another thread around Thanksgiving concerning table pads. Someone said they keep the pad the original size out of the package and tape it under the table. I tried this and it worked well. The best part was it didn't move around when putting on the tablecloth.