A series of polystyrene-based cationic copolymers (PSCC) were synthesized by copolymerization of styrene and the cationic comonomer vinylbenzyl trimethylammonium chloride. These copolymers can be used as internal paper sizing agents in a broad pH range. The effect of the charge density on the PSCC colloid properties was investigated. It was found that at room temperature the PSCC could change from water insoluble to colloidally dispersible and finally to fully water soluble as the charge density was increased. Based on the understanding of their colloid behavior, PSCCs were applied as internal sizing agents for paper. The results showed that the hydrophobicity of handsheets could be significantly improved by PSCCs and the sizing performance was a function of copolymer addition, charge density, particle size, drying temperature, and pH conditions. The present study indicated some potential advantages of PSCC copolymers as internal sizes. PSCC can be self-retained on the fiber surface, no hydrolysis occurs during the storage and application, and no sizing reversion occurs.