Precise control of width and length of one-dimensional (1D) semiconducting nanostructures has attracted much attention owing to its potential for optoelectronic applications. However, regulating both their length and width using conjugated polymers or even block copolymers is a huge challenge. To solve this problem, we synthesized a unique conjugated polyacetylene homopolymer by living cyclopolymerization, which spontaneously formed 1D nanoribbons via in situ nanoparticlization. Interestingly, their widths could be controlled from 8 to 41 nm, which were directly proportional to their degree of polymerization. Furthermore, a self-seeding technique via crystallization-driven self-assembly (CDSA) was used to control the length of the nanoribbons up to 5.2 mu m with narrow distributions less than 1.1. Interestingly, adding a block copolymer unimer to these nanoribbons produced triblock comicelles by the living CDSA mechanism. Finally, these nanoribbons were visualized directly by super-resolution optical fluorescence microscopy. Now, one can modulate both length and width of 1D nanoribbons simultaneously.