It really is a personal preference type thing,and in some cases depends on how much fabric you have.I have some that go the length of my table,with no hangover,and the width leaves about 2" bare on each side.I also have some that are shorter,but the width i keep the same,as that is the width i like.
Kathi

As I do not like my table runners to go over the edges, I make them about 2/3 the length of my table. But then again, my table is oval with the two leaves in it - not rectangular.

A width of 12" to 15" will cover the center nicely and not look too skimpy. A skinny runner would not look so good, IMO. I leave more than 2" bare table showing on each side of the runner- but that is my preference.

I see you are making this for your coffee table not your dining table. So I don't think I would have any drop down on the sides at all, but maybe leave 2" to 4" uncovered on each end. If you completely cover the coffee table then it's a tablecloth, right? *grin*

Thank you Teresa & Kathi,,,Yea,,I'm nixing the drop off the table,,I guess I was thinking about what they do for dining tables. Nicole