We have investigated size-dependent optical properties of poly-1,6-di(N-carbazolyl)-2,4-hexadiyne (poly-DCHD) nanocrystals in ensemble and in a single particle level. Our experimental results demonstrate that far-field microscopy coupled with an AFM setup is a reliable experimental approach for investigating optical properties of single organic nanoparticles. Single poly-DCHD nanocrystals show resonant Rayleigh scattering spectra with a lower resonance energy peak for nanocrystals having larger cross-section against its long axis. We also examined the extinction and the resonant Rayleigh scattering spectra in ensemble and the resonant Rayleigh and Raman scattering spectra and their anisotropy of single nanocrystals. The results indicate that the size-dependent resonance peak of the polymer-backbone electronic transition can be ascribed to structural confinement of nanocrystals, which reduces the effective conjugation length of π-electron in the polymer backbone for nanocrystals with a small cross-section.