"gigantic statue," late 14c., from Latin colossus "a statue larger than life," from Greek kolossos "gigantic statue," which is of unknown origin. The Greek word was used by Herodotus of giant Egyptian statues and by Romans of the bronze Helios at the entrance to the harbor of Rhodes. Figurative sense of "any thing of awesome greatness or vastness" is from 1794.
Helios, the sun, is a god everywhere; there was a scandal when Anaxagoras dared to call him a glowing clod. But the island of Rhodes is almost the only place where Helios enjoys an important cult; ... the largest Greek statue in bronze, the Colossus of Rhodes, is a representation of Helios. [Walter Burkert, "Greek Religion"]