Let’s say you have a Canyon Aeroad and could get exactly the same fit on a size S, M or L. What would be the most aerodynamic choice? All other factors, except frame, being equal.
I could imagine the bigger frame being more aero because you get more of the sailing effect. But I could also imagine the smaller frame being more aero simply because there is less bike to overcome wind resistance.
Smaller I’d imagine
I wonder - I guess you’re really looking at the headtube height - is it better with a larger headtube? Or worth more spacers? In a funny way - I would think smaller frames aren’t able to optimize airflow since they have such limited length to shape the air.
Would assume the largest sizes would have too much surface area
So somewhere in the middle? Total guess
First of all, you are not going to get exactly the same fit on all. Some pros use a frame that is a size smaller than what is recommended, because they can get into a more aero position — the seat post is higher and they can make their back flatter. But a more aero position is only helpful if you can actually maintain it. Most can’t for various reasons. Plus, other fit issues may arise. I have longer arms, which means I need a larger frame than my height would suggest (I’m 1.78 m on a size 56 cm with a 120 mm stem). Even now my legs are massaging my belly if I go full aero, which gets quite uncomfortable. On a size 54 cm, I would need a ridiculously long stem and for sure shorter cranks.