My gut sense is, the bigger (wider) the river then the more water you can effectively cover with a longer rod. It's not about casting distance so much as what happens after, during the drift. After all, on a big river with plenty of room behind, you can make a 100ft cast with a 9ft rod but do you get as much control over the cast as with, say a 12ft or 15ft?
Maybe someone can figure out the math and calculate how much additional water/fishing time per additional 12 inches of rod given a 90 ft cast?
Why not a 20ft rod if the river is big enough and you have the biceps to handle it - how about it Fred?