Yes, you could cut it in half. You could probably do thirds or quarters.
They grow just fine if placed on the substrate, in fact, they will throw their roots down into the soil and anchor. You must simply be careful to avoid burying the rhizome. This is the thick, horizontal part of the plant from which the roots and leaves grow. If that part becomes buried, it will usually rot and die, and kill the whole plant.
I'll attach a horrible Paint rendition of an anubias species. The black lines indicate an example of where I would cut if I had this plant.
They can also be attached to rocks. For holding them down to the substrate, you can use plant weights, stick small rocks on their roots until they anchor, or I've heard some people even use bent paperclips to "pin" them into the ground.
EDIT: Just realized you said the anubias was only 5-6". I only read the 12 leaves bit. So with a piece that size, I'd cut it in half, three at most.
