Maximum size for Lysiosquillina maculata is 15 inches, but there are many species of Lysiosquillids that it could be. Most common from the Indo-Pacific would be L. maculata, L. tredecimdentata, and L. sulcata. From the Altantic it would be L.glabriuscula and L. scabricauda. I have also seen a few L. lisa from Bali, but they are red and white striped. L. maculata is almost always yellow and black striped.
All of the Lysiosquillids that we know about are monogamous and live as pairs. They almost never leave their burrows, so if yours is out and about, it is probably because it is desperate for mate.
These animals are extremely slow growing as adults and may actual shrink in the aquarium. They live for years - at least 20.
Roy