Like many others here I have used both very short 6.5 foot rods to the longer rods in most the smokies streams.
My first fly rod I ever owned was a 6 wt 9.5 foot orvis. I agree the 6 wt was a little large for the smokies headwaters streams but the longer rod made for a much easier time as a new learning fly fisherman. Without a doubt the longer the rod the easier i find it to fish.
Personally, for the smokies headwater streams i like the somewhat longer 7.5 foot to 8 foot 4 wt rods. For the last four years I have always used my 7.5 4 wt Gloomis and my brother has always used a 8 foot 4 wt TFO.
unless you are picking a very specilized very very small stream rod I have found the longer rods much easier to control the fly.