When choosing a rug you should be mindful of the shape and size of your table.
You should be able to pull out all of the chairs from under the table and the chair legs should still remain on the rug. If your chair legs come off the rug then your rug is too small.
For round 6 person tables: 9x7 or an 8ft round rug
6ft table with 6 chairs : 8x10 rug
6ft table with 8 chairs: 8x10 rug
As a general rule: Add 24inches to each side of the table