My view of a fly rod length for fly fishing streams is, if my fly rod is longer than the stream is wide, go with a short fly rod, especially if it is for soft hackle and nymph fly patterns.
Being a Minnesotan, we make a frugal Scots Fly Angler seem like a spend thrift.
I bought a 4 wt, 7 foot, 2 piece fly rod from Cabalas for $40.
I have longer fly rods for open water, but for small streams I do not need to cast a country mile.
~Parnelli
"Everyone you meet in life, give you happiness! Some by their arrival, others by their departure!" ~Parnelli