hello Jenny. I had no idea myself so I searched a bit, and I found this article with very interesting guidelines: http://interiordec.about.com/od/chandelier1/qt/Dining-Room-Chandelier-How-High-To-Hang-A-Dining-Room-Chandelier.htm
hope it helps!

Rule of thumb is 36" above the table

More importantly, how large should it be? My thought is that 1/2 of the table top width is perfect. That allows diners to eat comfortably with their place setting out from under the light. They also won't feel like something is hanging over their heads.