I would write out the dimensions of your space on a piece of paper, including existing dining room table size and shape. Once you have this on paper and have added 2-3', as per ritasj's suggestion, to accommodate for the space around the dining room table, I would see what size of carpet you have yourself at. Do not be set on that size however, as it depends on where your purchasing your carpet from and in what shape. Manufacturers have pre-set sizes and you may have to adjust by a few inches to feet in width or length to accommodate. You will have a rough minimum idea of how big your carpet should be and can then have a look at what options are available for area rugs. If you are still not sure, I would then take a tape to your floor and mark out the parameters of the area rug to help give you a better visual of what size you may be ordering. Best of luck!