Regardless of how I might parse the OP's question, my answer is "the biggest tire that fits." The narrowest tire in my fleet is 32mm, because that's the biggest tire that fits on that bike. That one never sees gravel, and without fenders, it avoids wet roads as well.

My most comfortable bike, one that I've done maybe a dozen tours on is 650b x 42 with fenders. Since this thread is short on pics, here's "Big Red"


1973 Raleigh Competition, mods by Peter Weigle, paint by Ed Litton.