Re: What role does length play?
Its generally true that you can cast further with a long rod, but the reasons why are sorta complicated. A longer rod will keep your line higher off the water, especially when wading, and this sure helps you cast farther. Because its a longer lever, a longer rod should help you generate more line speed, helping you cast farther, but a good caster can probably cast further with a 5' rod than a beginner can with a 9' rod.
Longer rods help a ton with line control when fishing moving water where you're trying to reach over currents. They also generally work better for roll casting as well. Its also often easier for a beginner to feel a longer rod load, which can help them improve their timing.
Shorter rods are generally easier on the wrist, more accurate, and can make landing fish easier. (unless you bring the leader to line knots inside the rod tip and have a hard time getting them back out)
Many folks advocate short rods for fishing in heavy cover, but i often find evidence to the contrary. If you're going to be roll casting alot in that tight cover, you might be better off with say a 8'6" rod than a 7' rod. A longer rod in tight cover also gives you the option of dappling/reaching the fly over where the fish is with no cast involved whatsoever.
To wrap all that up, I think if you need the rod to reach over currents, roll cast long distances, or cast a long way, a longer rod will be a better choice. If you want to make short accurate casts, cast overhead in heavy cover, or feel light in the hand all day a short rod may be just the ticket.
But that's just me.