According to Colin Cowie, you should keep your centerpieces under 12 inches in height so that everyone at the table can chat with on another with uninterrupted views. Not only have I been preaching this for some time, it's just common sense.
True, there are tons of pictures with centerpieces that are higher than 12 inches. But you have to take into consideration, many of these are professionally designed to prevent as much interference as possible. On the other hand, some are poorly designed.
You can read more about these types of rules in my What Went Wrong Collection.