Three feet is generous; two and a half feet is enough... she is right about not catching the chairs on the rug when you pull them out to sit in them...

BUT that's 2.5 feet on every side. So you have to double that, and add it to the dimensions of your table.

SO... add 5 feet to 4 X 8, and you get 9 X 13...and if you fudge it a bit at the ends, which you can do, 9 X 12 is the right size. Some rugs are not exact, and if you find one that is 9 X 12.5, so much the better.

Thanks so much, you are right about the size. Boy, my math abilities are lacking!

You're welcome!
There's nothing wrong with your arithmetic...you got unclear instructions, and when you thought about it, you probably would have figured it out yourself.
It's just a matter of experience, my dear!

And, better too large than too small...nothing looks worse than a rug that is far too small for a dining room setup.

I agree. The table and chairs should fit comfortably onto the rug, with the chairs in the pulled-out position. Nothing says "skimpy" to me more than a rug that doesn't allow for the chairs to be fully pulled out.