The size of the printed photo is shown under each catalogue entry.
eg If you look at this "View over Aberdeen Harbour" photo, you'll see "Size: 10 x 6.3 inches.". So the printed photo is 10 inches wide by 6.3 inches tall.
Each photo is surrounded by a half-inch white border. So in the above example, the size of the card you'll receive is 11 x 7.3 inches.