The electronic and optical properties of quasi-one-dimensional transition-metal trichalcogenides remain largely unexplored beyond the mean-field approximation, and it is not clear how they evolve as the crystal is thinned. By performing first-principles many-body perturbation theory calculations, we determine the quasiparticle band structures and the polarization-resolved optical response of ZrS_3 in the bulk, bilayer, and monolayer limits, using a hierarchy of methods (hybrid DFT, GW, and the Bethe-Salpeter equation). We find that all three systems are indirect-gap semiconductors with a quasi-direct character, the direct-indirect splitting being -0.13 eV and nearly independent of both the level of theory and the dimensionality. The mean-field gaps are almost insensitive to thickness, whereas the quasiparticle corrections increase the gap by eV from bulk to monolayer. The effective exact-exchange fraction reproducing the GW gap scales as _^{-1}, increasing from to , so that no single hybrid functional describes the full dimensional series. The optical onset blueshifts monotonically ( eV) and the near-edge absorption is strongly dichroic. A change in light polarization does not simply shift the excitonic resonance ( eV); it also reorganizes the underlying excitonic wave function, selecting distinct band channels and k-space regions. A real-space, participation-ratio analysis resolves moderately delocalized, Wannier-Mott-like excitons templated along the covalent chains. They are predominantly sulfur-derived (75-88% of the density on sulfur for the chain-polarized bulk states), which identifies them as (S2)^{2-} disulfide excitations rather than S->Zr charge-transfer states, while in the bilayer the electron delocalizes across both layers, the real-space imprint of interlayer screening. These results provide a consistent reference for the quasiparticle and excitonic energy scales of ZrS3 from the bulk to the few-layer limit.