If you put your pedals at 9 and 3 o'clock, your riding shoe on the rearward pedal in its riding position and measure to the heel, measure how close that comes to your paniers, if you have them. The distance needed to clear them (if the heel hits) is how much minimum you'd need the CS's. You could also go with what most touring bikes have for CS length, which tend to be in excess of 17 inches, and probably be safe. Good luck
Zig