It's not perfect (the rounded edges don't help), but the best I could gather was:
Horizontal: 8.75 inches, 22.5 centimeters
Vertical: 14.375 inches, 36.5 centimeters
Note that the inches and centimeters were independently eyeballed from the same marks on a sheet of 17" (ish) paper with independent rounding so straight conversion and then averaging results might give you more useful measurements to go by.