A stock size intended to be added as a solution to the pulp stock in connection with the manufacture of paper or board in order to improve the characteristics of the paper or board, e.g.), to reduce the absorption of liquids, to increase the dry strength and wet strength. The stock size contains a polymer, which is based on styrene maleic acid and stabilised with a polysaccharide, such as cationic starch. The stock size is advantageously prepared in a steam jet cooker. The stock size is suitably used as a stock size in the manufacturing of liquid packaging board.