I feel like this was a big and hotly debated topic when I went on my cycling hiatus almost a decade ago, and we're still talking about it. It seems, as Eric mentioned above, that the smoother the surface the narrower and harder tire you want to run. You'll still see 23mm and narrower at the velodrome, but most riders are favoring wider tires out on the streets these days.
__________________
It's like riding a bicycle