Originally Posted by
repechage
There is a more recent favor of longer cranks than what was convention 30-40 years ago for a given rider, in general. But there are many variables as already suggested.
For a fixed gear conversion, a road frame to fixed, I would go 165mm as the bottom bracket will be lower than from a track frame, say, 70 mm of drop being very typical.
For a fixed gear conversion, it will be all about chainline. Probably driven by the rear wheel.
That gives me an interesting though, a mtb to 700c fixed gear conversion.. assuming you used a smaller frame size with 160 or 165 cranks OEM, you could slap on 700c wheels and possibly get away with 175 cranks and not have much concern about pedal strike..
As for your last point, assuming he uses a true fixed gear wheelset chainline is almost a non issue and will be more dependant on the cranks he chooses than the wheel but you're right, it's all about the chainline.