Sizes are randomised. Typically they are one of these:

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

If you are pre-ordering (i.e. reserving a future ant), you cannot choose the size - it is a randomised process.