size layer the total amount of applied size may be equal to the layer weight when size is applied using a furnish of less solids but in the case that the product basis weight is precisely specified e g at the above mentioned 80 g m 2 the amount of size may be increased making the product from a base sheet of 68 g m 2 basis weight for instance the application of a 25 solids size requires the size furnish to be applied by 24 g m 2 of the wet size film on both sides of the base sheet which is a rather large amount of applied size that may preferably be implemented using even a greater amount of size solids the invention makes it possible to optimize the consumption of raw materials and particularly the product stiffness as the size remains on the product surface the contribution of the size layer to the product stiffness is substantial whereby the product stiffness can be modified more easily by changing the applied size weight than by using a base sheet of different basis weight because the surface size is not necessary to increase the internal bond strength of the base sheet the properties of the size can be fully utilized to improve the surface qualities.