I would really like to see a pic. I have a reef lobster in mine. He is about 3-4 inches. He pretty much stays in his rock but if something new is introduced and gets to close to his place, he will snap his claws at them.
Sometimes you see XL domino or neon damsels that 'outgrew' peoples tanks. Large, fast damsels are what Id put my money on to survive a giant lobster.
Jon