Start with 28's- good, supple ones- at low-ish pressure and see how they work on your local version of chipseal. Around here, 25's are fine on chipseal but out in farm country the raods may not be as smooth to begin with.

Pretty much everything in the stable wears 25's with the exception of a new yet-to-be-shown all day rider that's wearing Continental 28's. And Schwinderella, which has Compass 650b 42's.

Ride 700 X 25's and 650 X 42's back to back and you'll discover how much slower the 650b's are. Cushy cushy and a lovely ride, but slower to accelerate, at least in my experience.