For dedicated commuting in the fast realm I lean towards 28-32 now. Did most of mine on 23 or 25. But I think the tiny speed advantage in speed you *might* gain on the skinnies is often outweighed by comfort, traction, and stability on something you can run lower pressures in.

Also, it's not all about size. You can get a good 28 that is lighter, smoother rolling, with better traction, than lots of 23s. I just ordered some jack browns in 33.3333 and fully expect them to be faster than some 25s I used to commute on. All while being much more comfortable, and able to drop way down in pressure when I'm playing in the dirt. (rarely)