Your dining room rug should be large enough to fit your table and chairs, but also offer enough buffer room behind the chairs so you'll never snag chairs or trip. If you're on the hunt for a new rug, remember the magic measurement: 91cm of clearance behind pushed-in chairs. Image from emilysnuffer.
That extra room gives people enough space to get up without catching their chair legs on the edge of your rug. When measuring to get the ideal dimensions for your new rug, measure your room, then your table. Then add 91cm from the pushed-in chairs around the table. Now you have the perfect size: everyone will sit comfortably, or be able to move around without tripping behind someone or when pushing in or pulling out a dining room chair. If your dining room is too small to accommodate this buffer space, you might want to skip the rug altogether.
Need-to-Know Numbers: the Dining Room [This Old House]