Two-dimensional (2D) polymer nanosheets have been attracting immense attention owing to their potential applications in optical devices, membranes, and catalysis. However, creating uniform monolayered 2D nanosheets through polymer self-assembly is very challenging, especially when using homopolymers. In this work, we designed a new crystalline polyacetylene that contains fluorenes and triisopropylsilyl side chains, which could self-assemble into sharp-edged 5-nm-thick square nanosheets with a narrow length dispersity of 1.01, by simple heating and aging in dichloromethane (DCM). Interestingly, the addition of tetrahydrofuran (THF) or chloroform to the heated polymer solution in DCM changed the morphology from square to rectangle. The aspect ratios increased linearly, from 1.0 to 10.6, according to the amount of THF or chloroform added, while maintaining narrow length dispersities less than 1.06. These unique fluorescent semiconducting nanosheets with tunable shapes exhibit high potential for optoelectronic applications.