Crystalline poly(3-alkylthiophene) (P3AT) nanofibers are promising materials for a myriad of device applications, but nanofiber length control and colloidal stability are difficult to achieve. We report an in-depth study of the solution self-assembly of regioregular poly(3-octylthiophene)-b-poly(dimethylsiloxane) (P3OT-b-PDMS) diblock copolymers with a crystallizable π-conjugated core-forming block. Use of the "living" crystallization-driven self-assembly (CDSA) seeded-growth method in solvents selective for PDMS allowed access to relatively low length dispersity, colloidally stable P3OT-b-PDMS fiber-like micelles with a crystalline, tape-like P3OT core, a PDMS corona, and lengths up to ca. 600 nm under optimized conditions. Significantly, the presence of a small percentage of common solvent and the use of slightly elevated temperature (35 °C) were found to enhance the length control. Analogous studies for P3OT-b-PS (PS = polystyrene) suggest that solvent composition and temperature represent key parameters for the general optimization of fiber formation by living CDSA for P3AT block copolymers.