8X10 inches - this is the kind of 20X25 centimeter that film competitions often require; 10X12 inches - also a common size; 11X14 inches - mostly the size of photo albums; 12X16 inches - general size of 135 negatives, complete in composition; 16X20 inches - the ultimate size of the ...
For many sizes, this implies that the ratios in the rounded off centimeter sizes are not the actual aspect ratios. For example, a 13x18 cm print is really 5x7 inches. This is a slightly different ratio and cropping to 13x18 ratio will give you a wrong ratio for the actual paper size....