2d8 for the bite, as standard for a Gargantuan creature (assuming the two sizes larger damage in the oversized maw ability), or take a hezrou's bite (4d4) and upsize it twice (6d6???)?
Fill in the x's...
Swallow Whole (Ex): A dire whiner can try to swallow a grabbed opponent up to one size category larger than itself by making a successful grapple check. Its jaws and guts have an extradimensional component that allows them to swallow objects far larger than their external dimensions suggest. Once inside, the opponent takes xdx+x points of crushing damage plus x points of acid damage per round from the demon's interior. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal x points of damage to the interior (AC x). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out. A Large obtestu's interior can hold 1 Huge, 4 Large, 16 Medium, 64 Small or 256 Tiny or smaller opponents.