This particular piece of wood is not a billet. This bookmatch appears to be about 8 inches across each side.
A billet is not an exact size - it's different for each process and each producer. We tend to chop 8/4 or thicker lumber into billets of about 10x24 to be able to handle them easier around the shop. For us, a billet is a roughly guitar-body-sized chunk of wood.
