A paper size which comprises a stable aqueous dispersion of a reactive size, from 5 to 100% by weight, based on the reactive size, of a substantially linear nitrogen-comprising polymer having at least 3 mmol/g of basic nitrogen atoms, and from 1 to 50% by weight of cationic starch having a degree of substitution of at least 0.05 is described. The paper size has a good sizing effect and a stable viscosity.