A quick note on the idea that Typhon, the primal Ancient Greek monster, was one possible source for the visual size of Cthulhu…
* is the right size: “a grisly monster” taller than a mountain. “His head reached the stars” — The Popular Encyclopedia, volume 6 (1841). Lovecraft describes Cthulhu as… “A mountain walked or stumbled”.
* is sort-of tentacular: described as having… “dragons’ heads on his hands instead of fingers” and “coiled serpents” for legs.
* has wings (in an early account): “he is also described with wings” — The Popular Encyclopedia, volume 6 (1841).
* is buried or sunken after defeat: buried or sealed either in the Serbonain Lake (Apollonius of Rhode, and also Herodotus), cast into Tartarus (Hesiod), or buried under the active volcano Mount Etna (Virgil). Tartarus is… “is a deep, gloomy place, a pit, or an abyss used as a dungeon of torment and suffering that resides beneath the underworld.”
* has historically been associated with volcanic activity: “In other accounts, he is confined [in] volcanic regions, where he is the cause of eruptions. Typhon is thus the personification of volcanic forces.” — Encyclopedia Britannica 1911. Newly formed lands at sea (“Dagon”, “Call of Cthulhu”) are usually brought to the surface by volcanic action.
* speaks an unfathomable and monstrous language: “In all his dreadful heads there were voices that sent forth every kind of unspeakable sound” — Hesiod. Also… “Whatever his form of utterance, his voice made the mountains echo.” — Gods, Goddesses, And Mythology, Volume 11 (2005).
* is alien: he is… “like neither gods nor humans” — Homer, in “Hymn to Apollo”.
* is cosmic: “Typhon terrorized the universe…” — Gods, Goddesses, And Mythology, Volume 11 (2005).
* is evil: “the murderous enemy of gods and men, the personification of physical evil, of death and destruction” — Bibliotheca classica: or, a classical dictionary (1833).