The site I'm going to link is a good one. He makes custom furniture and the section on tables (of all shapes) is a good one. You may have to search the site, but he used to have good advice on round tables. I wasn't able to find it quickly on his new site.

I'm a satisfied past customer. He used to post on the furniture forum but since this website has changed hands he's gone the same way of so many other valuable contributors...

Here is a link that might be useful: Paul Downs

I don't know about rule for sizes but I do know 3 ft from table for chair comfort. So if table were 4ft round you and chairs were across from each other you would want minimum 10 ft wide room for comfort of pulling out chairs.

Eandhl, I know the standard "rule of thumb" I was taught by designers was 24" for chairs; personally, I prefer your 36" suggestion. I think "rules of thumb" are often too minimal. I like to err on the side of "graciousness" when I'm able. I would also like to caution that any rug that will underlay a dining table should accomodate the expanded table by at least 24" on all sides. The rear legs of the dining chairs should remain on the rug at all times, esp. when the chair is set back to accomodate seating the diner!

When I ordered my sideboard from PD his website was different than it is now. At that time he had an excellent tutorial on round tables and the space they ought to occupy. I don't know if that bit of advice was culled when the new site was brought on line. I certainly hope not because it was very useful.

I think allowing 24-36" around the perimeter of a round/oval dining table is certainly adequate for the comfort of diners.