Usually, a table runner should hang over each side of the table where the ends fall. This is a lot more aesthetically pleasing to the eyes than a runner that stops where the table ends or is slightly shorter than the table. The amount of hang should be the same on both sides and can vary, just like the drop length of a tablecloth. But, at the end of the day, there are no strict rules, it's all about how it looks and how you style it. I've used shorter ones and they look pretty too.