Colossal is the biggest possible size in D&D system. A character or a monster is considered Colossal when he, she or it exceeds 64 ft. in height or length and weighs more than 125 tons. Such creatures usually take a 30 ft. square of place and have 20 ft. reach if they are long and 30 ft. reach if they are tall. The legendary Tarrasque is a Colossal Magical Beast.
A Colossal creature gets a -8 size penalty to Armor Class and attack rolls, a +16 size bonus to grapple checks and a -16 size penalty on Hide checks. A Colossal creature’s carrying capacity is 16 times as good as that of a Medium character (24 times for quadrupeds). A Colossal creature uses way larger weapons than a Medium character.
Those a bit smaller than Colossal size are considered Gargantuan.