Phillip,
As you mentioned, the size of the propeller that works best will depend on quite a few variables.
You can often use the smallest sizes with the widest variety of hook sizes. As long as the propellar fits onto the hook shank, it'll spin. It's when you upsize the prop that you'll run into problems with twisting.
I've always had the best luck/results by using the smallest propellar that I can get to fit onto the shank.
As Normand suggests, though, buying all three sizes and finding the ones that work best for YOUR flies is often the best course. The things are pretty cheap.
You don't even have to place them onto the hook shank. If you slide a bead onto the tippet, then the prop, then another bead before you tie on the fly, you can 'add' the attraction of the propellar to ANY fly (within reason, you wouldn't want one in front of a size 20 midge). I prefer this method, as the addition of the two beads offsets the propellar's propensity to rise in teh water column. If you choos this route, upsize your tippet one 'X' size to protect it from the abrasion of the spinning propellar.....
By the way, while some may decry their use, they are a time tested and tradtional 'addition' to flies. Most of my older tying books have sections on propellar flies, especially for streamers and bass flies. When discussing their use for trout, the comments are almost always about how effective they are on 'difficult' fish, for times when the fish seem to not be 'biting', and under 'difficult or adverse conditions'.
Plus, they are fun to fish. The fish tend to hit these really hard....
Buddy
It Just Doesn't Matter....