Depending on the maker, I use either a medium or large glove. In cycling gloves, I've needed large with both Pearl and REI brands. My Ergon grips are size small, which is perfect for me.

You're back where you started, more or less. Whatever works best for you, and as you note, you're not going to know for a while. Reviews do note that unless you have hands like a catcher's mitt, you are probably better off with small. I'll add my own experience - I tried the Ergons with the built in bar stabilizers. I did not like them at all. In fact, I cut down the Ergons with a hacksaw and added Cane Creek bar ends. Gets expensive, but it works well for me.