Let's do the math: You have a room 20' long. You need 3' on both ends to get by the chairs, so we lose 6 feet from our 20' and we're now down to 14'. Let's take away 18" for a buffet at one end of the room. You're left with 12.5' for your table, and I'd really prefer a smaller table unless you do a large number of humongous dinner parties, requiring seating for 14 people. Some say it's bad luck to have 13 at a table for dinner, but I'm not a believer in that. However, that may be why many tables seat a max of 12. I think a 10' table would work very well, and would indeed prefer it slightly smaller if you would remove a leaf or two to make it a more manageable size for dinner for 6 or 8.

4 " x 8" would work as long as this is the only furniture tou will have in the space and would be a much nice size.seating 12 in a pinch and 8 all the time I would go the larger one if it was me.

I would guess (technically) at least an 11x11 so that you would have at least 5ft. of space around the table. However, we have a 60" round table in our 12x12 breakfast nook and it feels very comfortable. We probably wouldn't have wanted it any smaller. So if you have a choice, I would say at a 13x13 or 13x15 space (especially if you'd like to include a console/credenza in the space).

10' wide 16-20' long