Make a place setting with plate, glass, cup and saucer and flatware. Then measure it and give a bit more for space. If you have a pretty 'regular' size table, lay a piece of fabric down the center leaving room for the place settings along side it and measure the fabric. These measurements would be a good start. Reminder, if you want to put something inside the placemats, don't make it so thick that a wine glass will loose it's balance!