use the chart in post
#5 as a guide, but before going narrower think about your objective.
I commute on 2" slick tires intentionally. The wider tire is more tolerant of lousy roads, potholes, concrete expansion gaps and sewer grates than narrower tires are. At reasonable pressure the rolling resistance is no higher (possibly lower) than a narrower tire would be.
The wider tires are especially nice at night when road hazards are harder to see.
I'm not saying 2" is absolutely right, but something bigger than 1.5" is more suited to commuter use than narrower tires are.