How is flow separation related to the chord length of an airfoil?
The chord length is a minor influence. The main driver for separation is the local pressure gradient.

Make sure to read the first three paragraphs of this answer if you are unsure how the pressure gradient drives separation.

The chord length only comes into play via the local Reynolds number. Higher Reynolds numbers mean steeper pressure gradients can be tolerated, but the effect is rather weak. It shows in the separation point which moves back slightly on the upper side of an airfoil near stall at the same angle of attack when the Reynolds number is increased.