The sign for "giant" will depend on what you are talking about.
If you mean "giant" as in a fairy tale type or giant, then you can exaggerate the sign for TALL by using a bigger movement, eye-gaze, and a more intense facial expression (or even the mouth movement that looks like you are saying "CHA!"
This sign is particularly good to use for describing people but not good for describing buildings. This version of TALL means "very tall" and if you were telling a fairy tale of some kind this sign in context would mean "giant."You can do this sign with less facial expression and not so high up and without the upward eye-gaze to mean just "tall" rather than "very tall."
very-TALL (basketball player height or "giant")
If you mean "giant" as in "big" see: BIG
Notes: