26x1.95 on my "hybridized" 1995 Specialized Rockhopper Rigid MTB/Single Speed with fenders
Honestly, though, my girlfriend is riding 700x35c tires on her Trek 7100WSD and has never had issues. If its fine gravel thats packed, I would say 32 would be fine as well. If its bigger gravel that can be loose at times, go with 35-38c
I see guys with road bikes (23 or 25c) going buy all the time on the canal path. I wouldn't do that, the path isn't THAT smooth!