Photon-correlation measurements were applied to single chains of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] of various molecular weights in a fluid solvent in order to investigate the relationship between the probability of single-photon emission and spatial size of the chains. We clearly observed that polymer chains with faster translational diffusion time exhibited a high probability of single-photon emission. This result indicates that the photon antibunching behavior of single conjugated polymer chains is strongly correlated with their spatial size, and that the polymer chains can be made to behave as single-photon sources by controlling the size.