Silk-elastin block copolymers have such physical and biological properties that make them attractive biomaterials for applications ranging from tissue regeneration to drug delivery. Silk-elastin block copolymers that only assemble into fibrils at high concentrations can be used for a template-induced fibril assembly. This can be achieved by additionally including template-binding blocks that promote high local concentrations of polymers on the template, leading to a template-induced fibril assembly. We hypothesize that template-inducible silk-fibril formation, and hence high critical concentrations for fibril formation, requires careful tuning of the block lengths, to be close to a critical set of block lengths that separates fibril forming from nonfibril forming polymer architectures. Therefore, we explore herein the impact of tuning block lengths for silk-elastin diblock polypeptides on fibril formation. For silk-elastin diblocks E -S , in which the elastin pentamer repeat is E = GSGVP and the crystallizable silk octamer repeat is S = GAGAGAGQ, we find that no fibril formation occurs for = 6 but that the = 10 and 14 diblocks do show concentration-dependent fibril formation. For = 14 diblocks, no effect is observed of the length (with = 40, 60, 80) of the amorphous block on the lengths of the fibrils. In contrast, for the = 10 diblocks that are closest to the critical boundary for fibril formation, we find that long amorphous blocks ( = 80) oppose the growth of fibrils at low concentrations, making them suitable for engineering template-inducible fibril formation.