Originally Posted by
bikemig
The obvious answer is that there is no such thing.
Agreed. Most of my gravel road touring has been on 23mm tires mainly since the bike I use most doesn't accept anything wider. Worked fine and no problems with the tires (did need to make sure the pressure was high enough). I've also done some touring with another bike that currently has 35mm tires, but I'm not sure any of it has been on gravel.