I canít speak for the Missouri, but I am ĄTrout Speyingď and swinging a fly all year round in my home waters in Bavaria.
I guess trouts as a species are not that different, no matter where they are targets for us.
Most of the time I use all kind of Streamers, while the singlehanded equipped fishermen use the smallest nymphs.
But there are some weeks in autumn, when I have better results with smaller streamers or soft hackles.
This is especially the case in days or after days of more fishing pressure, mostly in days of nice and calm weather, letís say in fishing days also for the less passioned... More people fishing.
In my experience it turns again in bad weather conditions. The indicator in choosing the fly size for me is fishing pressure.
Also done by myself of course, after presenting all my big patterns to the same trout...
Time is running... A little bit of work, a little bit married, fishing, hunting... What else?