Please note that the size stated pertains to the size of the mount not the image/print size:

28x20ins (70x50cms) mount with an 18x12ins (45x30cms) central image/print.
16x12ins (40x30cms) mount with a 9x6ins (22x15cms) central image/print.

The mount sizes are the same as the metric frame sizes you go on to purchase for your mounted prints - be they standard shelf selections from high stores or custom made from your local art framer.