Metal Sonic Kai is a giant transformation of Metal Sonic after he has absorbed the energy of the Power Gems on Carnival Island. It appears as the final boss of Knuckles Chaotix. It stands at 20 feet tall, is red and has a monstrous face with a gaping mouth full of fangs. Its arms and legs also function separately from its body, increasing its attack range. It attempts to strike its foes with its un-attached arms and blast them with a powerful laser fired from the cavity in its torso.
Titan Metal Sonic
In Archie Sonic's Chaotix tie-in special, Metal Sonic absorbs a Power Stone to become a giant. Unlike its game incarnation, this version's limbs are actually attached to its body. Knuckles the Echidna was able to use the Power Stones' energy to make himself giant as well, and was able to KO Titan Metal Sonic easily.
In Archie Sonic's 231st issue, Dr. Eggman deployed a new Titan Metal Sonic from his Death Egg Mk. II. The huge robot rampaged through New Mobotropolis until it was struck by a crystalline spell cast by the evil wizard Ixis Naugus. The robot was transformed into a crystal golem which Naugus could command and it used its body to shield the city from the Death Egg's annihilator beam. The golem was destroyed and the city saved.
In the UK-published Sonic the Comic, the Emperor Metallix is the leader of the Brotherhood of Metallix. While based on the Metal Sonic Kai from Chaotix, it is very different. It still resembles the standard Metal Sonic/Metallix, but with a red body that is fused to a hovering throne. It was not designed for combat, rather it was originally created to oversee the creation of the ultimate Metal Sonic robot. However, it went rogue and led its army of Metallix drones towards the conquest of Mobius by tampering with the timeline, attempting to erase Dr. Robotnik from history so that the doctor could not develop the Metallix self-destruct program. Thanks to Sonic the Hedgehog and the Chaotix Crew, history was set right and Dr. Robotnik used the self-destruct system to destroy the Metallixes when they attacked his citadel.