To begin measuring, arrange your dining room furniture in the exact layout that you want. Then, measure the table top and add 24-30 inches to every side to account for chair space. You definitely don’t want the rug to be too small that the chairs end up scratching your beautiful hardwood floor and ruining the ends of the rug.
While you want the rug to be a large enough to act as a cushion underneath the table, you also don’t want it to be too big that it almost touches the walls. Create some distance (at least 8” on each side) between the rug and the walls, plus any other furniture you have in the dining room.