All forms are measured in inches.
Stacks with 8x13.
The forms are wood so to make them last for thousands of uses make sure you let them dry in between each use. The timing of making them and removing the form from the slab is very similar to plaster. If left on too long your clay will crack, the wood will mold or start to get soft. Mold can easily be washed off with dish soap and if edges get rough you can use sand paper to smooth them.