How to measure a frame to find an appropriate rim?
A 700c wheel has a 622mm bead seat diameter, which would mean a 311mm radius. 

But note that, according to Sheldon, "The rim's diameter will generally be 5-10 mm larger than the bead-seat diameter, depending on how high the rim flanges stick up above the bottom of the rim channel."  So when Paparazzi measured a 12.5 inch (317mm) radius on a 700c wheel he was seeing 6mm larger than the bead seat radius, which is in the expected range.



Roughly speaking, the brake pads for rim brakes should be positioned at about the bead seat (so as to fall on the flat of the rim), so for a 700c they would be roughly 311mm from the center of the axle dropout slot