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.

It's better to have a light that is slightly too big than one that's too small, plus the structured and open design of the Restoration Hardware means it doesn't take up as much visual space as other designs might. I think the Restoration Hardware pendant is actually the better fit for your table--the Ballard one risks looking too small against the whole table, whether or not the leaf is in it. If you did decide to go for the cool industrial black and brass chandelier, I'd recommend buying two, imagining your table divided evenly in half and then mounting each chandelier so it is centering its half of the table.

Here is a picture of the space. I didn't want to share because of all of the personal photos but I figured out how to blur them. There seems to be a lot of visual space to work with but not much physical space.

Chandelier size is length of room + width of room. So a 10 x 14 room = 24 so 24" chandelier. Tat lot is beautiful and pricey