If your sofa and armchairs are against the walls,your rug should be big enough to go under the front leg of all large pieces (red outline) or, if you would like more of your floor to show, small enough that none of the furniture legs overlap it (lavender outline).
If your furniture is floating in the room, the ideal rug will be big enough to go under all four legs of each large piece (red outline) or again, if you prefer to see the floor, small enough that none of the seating pieces sit on it (lavender outline).
There are three different ways to measure a rug for your dining room. First, make sure to maximize rug so chairs clearly stay on rug even when pushed out. Second, for a smaller room, see that front legs of chairs stay on rug 6" when pushed out. And last would be somewhere in between.