I think there are so many variables that it would be impossible to find a formula to suit all hulls. The important thing is to gave a rudder in line with the prop shaft so that the prop wash goes each side of it, and if feasible to have a partially balanced rudder - say with the shaft a quarter to a third back from the leading edge. Depth is better than length, except in sailing barges. If the sunseeker is a fast motor cruiser, I'd try a rudder as deep as the prop diameter, and about 4/5 as long, 1/4 balanced. If you are only using one shaft, just have one rudder - a rudder not in the prop wash won't do much.
I think I've just put myself up as an Aunt Sally but hey, have a go, boys!
