http://ezinearticles.com/?How-to-Mak...Set&id=2088576
In summary:
For Single Bed- size: Runner Length = Bed width + Drop from Mattress-top to Floor. Runner Width = Approximately 18" to 24"
For Double Bed - size: Runner Length = Bed width + Drop from Mattress-top to Floor. Runner Width = Approximately 18" to 24"