You really need to look at what type of fishing you will be doing, even in the park. Small, rhododendron-choked streams will be easier to fish with a short rod, but in general, the longer the flyrod you can get away with, the better job it will do of controlling your line in the mixing currents. The old locals were experts at long cane poles and high-stickin'. There is a lot to be said for that approach in the park. The more line you can keep off the water, the better.
My preference is an 8'6" 3wt (sometimes I wish I had a 9'6"), but your best bet is to find what works for you.
"Even a fish wouldn't get into trouble if he kept his mouth shut."