Paper size mixtures comprising a reactive size dispersed in water and a polymer, which are obtainable by mixing (a) an aqueous dispersion of a reactive size whose dispersed particles have a mean particle diameter of less than 500 nm, the dispersion being obtainable by emulsifying at least one reactive size in water in the presence of at least one surfactant, and (b) at least one emulsion polymer and/or at least one water-soluble polymer which does not have a dispersing effect with respect to reactive sizes and/or a water-dispersible polymer, and use of said paper size mixtures for the engine sizing and surface sizing of paper and paper products.