Well, I use .093 for everything. Big knives, small knives, medium sized knives. All of the sheaths that I make, however, are built for inverted or "any direction" carry. They're usually for harnessing rigs or modified "static cord" rigs that I modified from Blade Rigger's static cord concept. I've found that .093 works best for creating enough hold while still permitting the knife to be snapped in and snapped out of the sheath. Unfortunately, it can also be the hardest to shape precisely compared to .060.
Basically, kydex is so forgiving that even after a little practice with a couple of different thicknesses you'll find which kydex works best for which knife.
Matt