650c has become an exotic size now. It is still used in France but almost disappeared from french shops and suppliers as "randonneuse" and "cyclosportive" had become a kind of antique bikes when the fashion was big swing for the MTB.
It is wiser to make a road bike in 26" than a 650 size almost any store stock this size.
There is a small trend with the 650 randonneuse style but the market is tiny for sport type 650.
Looking at the tires makers catalogs and it is easy to confirm my thoughts.
Almost no tires are available for road use in 650c sizing (650x20 or 23 etc...)
But looking at 26" there is much more choice of width and pattern.
26" Vs 700c
Due to the exotism of the 650c I concetrate my work on the 26" sizing.
Why 26" instead of 700c
26" got advantage for mountains / alpine style road (hey, it is why MTB use this size at the beginning right?)
It is not only goof fit for small rider, it is also more performant for hill climb and and cornering down the roads.
Also, shortest spokes provide stiffer wheels.
And finally, smaller rim diameter and shorter spokes means less weight in rotation.
I noticed several times that while going down Norikura skyline road and other alpine roads at over 100km/h the rear wheel has not much grips when braking hard before cornering.
Mounting 26" offer a choice of high pressure clincher with wider section and still light as 700c clinchers