The size of all Filofax diary and paper refills (e.g. pocket, personal) is printed on the left hand margin of the paper, together with the product reference code.
The product reference code for Filofax diary refills changes every year to reflect the year that the diary begins, so if you have a diary that starts in 2011 the product code will end in 11 (e.g. 6842411). The following year’s diary will carry the same product code but ending in 12 (e.g. 68424112).