This is really nightmarish ... like something out of a HP Lovecraft novel. Or in D&D lore, perhaps a boss-type from the Illithid empire. Really well executed from your concept artwork. It does look rather colossal. How big compared to your other slugs and snails?
Its quite a massive beast going around 120/150mm lond ( depending if you measure just the body or body+tentacles ) and 70/100mm tall.
That WH peasant just doesn't have any luck...
he does seem to find its way to the next monster on the list though.