Interesting read. The "new" oros indicators are the bomb... for real. Worth their weight in gold. I only use the smalls, but paired with any nymph rig, I am confident in detecting a hit. Or for using something tiny i'll tie up a tiny polypropylene indicator.
For me, the depth of water, weight of the nymph rig and length of leader determine what size. Typically go back and forth between a 1/2 & 3/4. Use the smaller ones for light rig or really shallow fishing