I suspect that the upper limit for fishable size is somewhere in the size range of a herring (4-8") and I've caught coho from the beach on clousers nearly 4" long, but a fly that large is not fun to cast. Try ~2-3" total length with a sz 1 to 2/0 trailer hook.
I tie up a variety of lengths from about 1.5-3". The little ones can always be used for SRC's. I also tie almost all of mine with trailing/stinger hooks now. Might just be my imagination but I seem to have better hookups with them.
Mine usually end up in the 2-4" range with most around 3" probably. Clousers with stingers or lately tube flies with a cone but the same material and colors as my clousers. I am finding the cone flies cast much better than a clouser and a tube fly with a cone is easier and faster than a clouser with a stinger. I don't think they fish quite as jiggy though. Might need some heavier cones.
I have seen sea run Cutthroat take some pretty surprisingly large bait at times. Here is a picture of a Sculpin that is just over 3 inches long. I have seen them choke down full herring of well over six inches long. http://olympicpeninsulaflyfishing.blogspot.com