Being the only one of his kind it is safe to assume that Protoform X stands over 10 feet tall if not more.
Rampage as he is otherwise known as, has an experimental spark that is almost indestructible and can only be damaged or destroyed by an Energon blade. Even if he is damaged to the point when he is in several pieces as long as his spark is undamaged, he will rebuild himself.
This invincibility is gifted to him at the cost of his sanity, as he is psychotic in every way. Unlike all other Transformers he feeds on the emotions of others, especially negative ones like pain (both emotional and physical) and fear; he is also a cannibal having eaten his creators (an entire colony of Maximals).
Like most Transformers he has an alternate mode, although unusually he has three modes like a Triple Changer; Humanoid, king crab and a third mode that resembles a tank. He is also a transmetal making him even more powerful and hardy than an ordinary Cybertronian.
After his creation and the slaughter of his creators he was eventually subdued and transported to a space station to be executed - there it was found that most conventional weapons proved ineffective against him and was later queried whether or not his spark could be terminated.
He eventually broke out of there too, killing and eating many of his captors before again being captured by Depthchargeand put into stasis. Depthchagre took him before the Maximal high council and requested to personally end Protoform X's existence but the council denied this and stated that he be kept in permanent staces.
He was placed on board the Axalon, a Maximal scientific vessel, but was inadvertently released from his slumber when the Predacons attacked. After a series of events in which he blasted his way through both factions without signs of stopping and tore Tarantaluslimb from limb, he was blown off a cliff face and lay unconscious until Megatron(The Predacon leader of no relation to the Decepticon) cut out a portion of his spark using an energon knife and renamed Protoform X as Rampage (after his actions on the day).
Under Megatron (who held him prisoner, crushing the portion of the spark whenever Rampage showd signs of disobedience) Rampage fought many battles with the Maximals until a grudge-bearing Depthcharge (who had had friends on the station where Rampage broke out of stasis) arrived on the scene.
The two had many battles of their own, both of them surviving massive damage including Rampage being impaled and Depthcharge having a piece of his back ripped off. Eventually Rampage was killed by Depthcharge when he grabbed a solid shard of energon and stabbed Rampage through his spark, although the resulting explosion killed them both.