Gunwale, IME. Its the highest point and therefore the quickest to create interference. 240cm is pretty short for most canoes, 280 is pretty long for solo hulls.
Stroke angle matters, torso length matters, and seat height matters, but most people seem to like how a 260 feels. I have two 280cm and I only wish I had a 260 when I double-blade paddle my Encounter with a 27" GW.