This invention is directed to novel sizing agent compositions containing urea which is chemically modified and/or combined with other materials which produce desirable sizing properties. These novel agents are used as beater additives in pulp refining for the manufacture of cellulosic products or are applied to finished cellulosic products as surface sizes. The invention is also directed to methods of making the sizing agents. In one preferred embodiment, urea is reacted with sulfamic acid and the resulting reaction product is combined with rosin and further reacted to produce a novel sizing agent.