All forms are measured in inches.
In most cases this size stacks above the 6x15 Oval to create double depth. The geometry of the cut and batch it was cut in creates could create some minor seam fixes when your piece is completed.
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.