My grandmother was a weaver. She made her place mats a bit larger than usual. I can no longer remember the size she made, but I remember her 'formula'.
She set the table, placed a plate, glass, napkin etc down on the table. Then she sat and checked if the placement of things was good. She wanted the glass to be on the place mat, and there to be room for a wider plate and place setting. I believe hers were at least 15 inches tall by 20 inches wide. Larger than usual, but they really worked well.