A paper sizing composition, comprising at least one first sizing component dispersed in water, the said sizing component being prepared of a reaction product of a maleic anhydride and a fatty acid component, and an aluminium component. The fatty acid component comprises an alkyl ester of one or more fatty acids selected from a vegetable oil based fatty acid mixture. The particle size of the particles in said first sizing component has been reduced so that in the dispersion at least 50% of the particles are <2.5 .mu.m, at least 75% of the particles are <4 .mu.m and at least 90% of the particles are <5 .mu.m, preferably so that at least 50% of the particles are <2 .mu.m, at least 75% of the particles are <3 .mu.m and at least 90% of the particles are <4 .mu.m.