When I enter a room one of the first things I notice is the balance (or lack of it) in the room. I see so many area rugs that are undersized or that the furniture is not positioned properly on the rug.
When choosing a rug for under a dining table the rug should be wide enough so that when pulling out a
chair the chair should remain on the rug. This will also insure that when seated all four legs of the chair will remain on the rug. A rug allowance of 24" beyond the width of the table is usually sufficient.
For example: If your table is 44" wide, allow 24" on each side of the table for the chairs. That gives you a rug width of 92" or approximately 7 1/2 feet. Your calculations may fall in between sizes. Since rugs come in standard sizes, unless it's a totally custom cut rug, remember it's better to go larger than smaller.
Use these same guidelines to determine the length of the rug.
When browsing furniture stores and/or magazines sometimes you'll see furniture angled on a rug with one or two of the front legs on the rug and the other legs resting on the bare floor. In some instances this may be acceptable, but, generally keep the entire piece of furniture on the rug or totally off the rug. It looks a little more pleasing and it keeps the furniture more stable.