If you're looking for a change a rug will really make a difference. I think it's pretty common to have a rug under a DR table but you don't have to.
For size, a good guideline is, there should be enough room to pull the chairs out from the table and all legs still be fully on the rug

I have a rug under my round kitchen table and I really like the look.
old pic.
{{!gwi}}

I would've had problems using a rectangular rug because of the dimensions of the room (rug would hit the wall) so I went with a round. I actually think I really would've liked an 8' square rug too.

Wanted to add that I have a dustbuster handy to quickly vacuum up the mess and crumbs.

I do expect to find a rug under a table in a dining room. In a kitchen, not so much, but in general I always think a rug under the table makes the room appear more "finished." As Shee said, the rug needs to be large enough so that when you pull out the chair to sit, the chair stays on the rug. I don't have a rug under my kitchen table at home (mainly because it's very close to the family room sitting area, which has a rug, and the two rugs would be almost touching each other) but I just put a rug under the table in our summer cottage and it looks so cozy. I bought a natural fiber rug (I think it's seagrass) from Overstock and lucked out, as it was cheap and ended up being a good quality (Safavieh) rug.

About a week ago I pulled up my rugs in the kitchen, dining & living room. The wool rugs in the kitchen & dining area shed horribly. Kids are grown so that's not an issue for us and I prefer the clean lines of no rug. But we're supposed to have company this coming weekend so now I'm planning to put the living room's synthetic rug on the hardwood under the dining room table; it seems to anchor the table and protect the floor. So, I change my mind with the situation. Obviously my husband is a dear as he's the one challenged with hauling the rugs around.

I think tables *look* better with rugs underneath. We don't have one under our kitchen table (small...in a breakfast nook) or under our dining room table (full size, but we eat every dinner there). I have three small children so it's just not practical. Oh the messes the make down below, ha ha. So if your children are older now, I say go for it. I bet it will make a big impact and look wonderful!

When our children were older, I used a sisal rug with a painted pattern purchased from Ballard Designs. I had one under the table and the U-shaped kitchen area. Both held up very well. I sprayed the kitchen rug with a protection coating. Didn't change it at all and helped keep it cleaner looking longer.

We're empty nesting now, and I have a flat weave Soumak under both the breakfast and the dining room table. Both can be seen in the link below:

Here is a link that might be useful: Soumak rugs

I am debating on getting another rug for under my kitchen table. When we first moved in I bought one on clearance at bb&b. I really liked the look but not the staining.. However, I have been told when I do buy a new one for the kitchen an indoor/outdoor rug is the best bet. Ballard designs has a ton of them for indoor/outdoor that they claim are stain resistant. That might be your best bet. Prices are fairly reasonable IMO too.
good luck!

I added a regular wool rug under our breakfast table (a cheap one from Overstock). Wasn't sure if I would like it or not (we have kids who are 10, 7 and 4). It looks nice and has held up surprisngly well. The cheap is good though; don't have to worry when dyeing eggs or doing crafts.