The present disclosure relates to a method for controlling size distribution of formed polymer particles during preparation of aqueous polymer dispersion. The method comprises obtaining an aqueous polysaccharide solution, which comprises about 10 to 40 weight-% polysaccharide containing a free hydroxyl group, such as starch, and polymerising in the polysaccharide solution in the presence of a graft-linking, water-soluble redox system about 30 to 60 weight-% of at least one optionally substituted styrene, about 60 to 30 weight-% of at least one C1-C4-alkyl(meth)acrylate, and 0 to about 10 weight-% of other ethylenically unsaturated copolymerizable monomers. The size distribution of the formed polymer particles is controlled by adjusting viscosity of the polysaccharide solution before the polymerisation to a level less than about 20 mPas.