Giant Pokémon are Pokémon which are significantly and abnormally larger than the normal size specifications. There have been several occurences of giant Pokémon in the anime.
Examples of Giant Pokémon
In the episode IL013: Mystery At The Lighthouse, a giant Dragonite appears towards the end of the episode, where Bill is searching for it.
In the episode IL019: Tentacool & Tentacruel, a Tentacool evolves into an abnormally large Tentacruel after exposure to Team Rocket's
Also in the episode IL017: Island of the Giant Pokémon! some giant Pokémon can be seen such as Pikachu, a Blastoise, etc., although at the end of episode it is revealed that they were mechanical replicas of a theme park.
A giant Gengar and Alakazam, as well as a giant Jigglypuff were also seen after Ash Ketchum discovered the ruins of an ancient city in IL072: The Ancient Puzzle of Pokemopolis.