The alpha-helix to beta-sheet transition (α-β transition) is a universal deformation mechanism in alpha-helix rich protein materials such as wool, hair, hoof, and cellular proteins. Through a combination of molecular and theoretical modeling, we examine the behavior of alpha-helical coiled-coil proteins with varying lengths under stretch. We find that the occurrence of the α-β transition is controlled by the length of constituting alpha-helical proteins. In the asymptotic limit, short proteins with less than 26 amino acids or 3.8 nm length reveal interprotein sliding, whereas proteins with greater lengths feature an α-β transition, leading to a significant increase in the protein's stiffness, strength, and energy dissipation capacity at large deformation. Our study elucidates the fundamental physics of this mechanism and explains why the α-β transition typically occurs in protein filaments with long alpha-helical domains.