Some friends of mine ride a tandem on gravel rides and recently switched their 700-size wheels with some form of 650? size wheels with big tires. He said the OD of the tires was the same as with the 700's. Not sure exactly what rims/tires he is using to do that. The tires looked about 2" wide.

Edit: I think it's the same size as on this bike- the 780 may be rim diameter, 650b wheel size...I think:
http://www.feltbicycles.com/USA/2014.../7-Eighty.aspx