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.

I have a 54" round table that I love. We use it with 4 matching chairs - so I can't really answer your question. BUT I can tell you that finding tablecloths for this size is difficult. 70" is too big and 90" is too small. If you love tablecloths, be forewarned. :) But 54" is a great size in all other respects!

Edited answer: For the rug, add 12" to each of the four sides, so a minimum of 9.5 feet square, but 10'x10' would be even better. General rule for table top space is 24" per diner. On a square table, you also have to account for overlap, so your table should fit eight comfortably.

Ideally allow at least 3 feet for each chair (2' chair depth + 1' to get in and out). So if you have a 12' space, 6' might be taken up by chairs. That means you can fit a 72" round but wouldn't have a lot of extra space. The leaf would make it really tight. I'm all for extra space, but I fell in love with a round table, so I will deal. I have an 8x8 space plus walk room on one side, and I picked out a 48" round table. I have no extra room on some sides but the roundness helps. There are a lot of options. I would go into a showroom and ask the sales associate for help. There are convertible tables, thin pedestals, variable tabletops, exterior and interior legs, 3 legged tables in addition to leafs. Sorry that was probably not much help. I went into a crate and barrel showroom and asked for help and I was able to figure out what I wanted through the process of elimination. They have enough different configurations that you can get an idea of what will and will not work in your space. I would optimize floor space for your family and only add the extra tabletop space when you absolutely have to. The chairs make a big difference in how much space you have around the table when not in use. You can choose uncomfortable chairs that fit tightly against the table (if the table allows) or comfy upholstered chairs that take up more space when pushed in as much as possible. If you go rectangular, you have the option of benches. They have upholstered benches too. Every inch matters. If you can trim a few inches off the chairs or table edges or scooch the chair under the table a bit more, you may be able to come up with another 6-8" around the table. This one says it will fit 6-8, but I wonder if you could get more kids around it if you use benches. (It's not the most attractive IMO.) Table goes from 34x57 to 57x68.