Formal:
* Très grand: This is the most direct and common translation.
* Immense: This implies a larger size than "très grand" and has a more powerful connotation.
Informal:
* Énorme: This is a very common and informal way to say "very big" and implies something incredibly large.
* Gigantesque: This is a more poetic and literary way to say "very big" and suggests something truly massive.
Specific contexts:
* For a person: You could also say "très costaud" (very strong) or "très corpulent" (very corpulent) depending on the situation.
* For a quantity: "Beaucoup" (a lot) or "une grande quantité" (a large quantity) could also be appropriate.
Ultimately, the best translation for "very big" will depend on the specific context and the intended meaning.