Size control has been successfully achieved in inorganic materials, but it still remains a challenge in organic polymers due to their polydispersity. We here report a facial approach to tailor the diameter of polyaniline (PANI) nanofibers in a range of 200-30 nm via temperature-controlled polymerization from room temperature to -192 °C. It is shown that the formation of PANI nanofibers was directed by the self-assembly of an amphiphilic aniline-glutamic acid complex, which formed micelles with different sizes at various temperatures during polymerization. The size-dependent properties of the resulting PANI nanofibers, such as molecular weights, optical properties, crystallinity, and electron conductivity, have been discussed. Most importantly, we showed a more than 3 magnitude increase in the conductivities of doped PANI nanofibers with a decrease in the diameter from 150 to 30 nm. A homemade sensing device was constructed, and it is shown that PANI nanofibers with smaller diameters exhibit a much faster response than those of larger-sized fibers or bulk PANIs due to their large surface area and high intrinsic conductivities.