Sizes are randomised.
Typically they are one of these:

6 x 8 inches (15.2 x 20.3 cm)
8 x 10 inches (20.3 x 25.4 cm)
9 x 12 inches (22.9 x 30.5 cm)

If you are pre-ordering, you cannot choose the size, it is a randomised process.