Heavily weighted streamers, floating line and long leaders — 6 or 7 —but tiring, especially in windy conditions
Unweighted streamers, long sink tips and short level leaders - 5 - not tiring and more effective for distance and accuracy
You don’t need a 6 or 7 to handle big fish, you need it to cast unwieldy, heavy flies
A five weight will and has (see below) handled a lot of big Montana trout with unweighted flies.
My two go to 5 weight rods for SW Montana streamer fishing are:
T&T NSII with a 200grain SONAR sink tip and Sage XP 905-2 with a 150grain SONAR sink tip.
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27”+ Brown on XP with unweighted #10 bugger
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Lower Beaverhead brown on T&T NSII 5 weight with #4 Pine Squirrel bugger
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21”+ Brown from Gibbon Meadows - T&T NSII 5 weight and soft hackle streamer
YOU Don’t need big rods to handle big fish. IMHO