Translation guide
The English word 'giant' can be a noun meaning a huge mythical creature or a very large person/thing, or an adjective meaning extremely large. Japanese uses different words depending on whether you mean a mythical giant, a large person, or something of enormous size.
mythical giant
giant (adjective)
A huge human-like creature from myths and fairy tales
A person, animal, or object that is unusually large
Describing something as enormous in size, scale, or degree
The most common adjective meaning 'giant' or 'huge'. Used for physical size, scale, or impact.
A giant building is standing.
It had a giant impact.
Means 'extra-large' or 'giant-sized', often used for products or portions.
I ordered a giant pizza.
Casual adjective meaning 'huge' or 'giant'. Common in everyday speech.
Very casual, emphatic way to say 'ridiculously huge'. Often used for comedic effect.
巨人 is a noun meaning 'giant' (mythical or figurative). 巨大な is an adjective meaning 'giant' or 'huge'. Do not use 巨人 as an adjective.
A giant appeared.
We found giant footprints.
The loanword ジャイアント is mainly used in proper nouns (e.g., ジャイアンツ baseball team) or specific product names. It is not a natural way to say 'giant' in general Japanese.
He is a giant of a man.
Describing a very large, strong man
a giant corporation
a giant step forward
Figurative use; 'giant' is often translated as 大きな (big) in such contexts
A giant appeared in the old tale.
でかいクモがいた！
There was a giant spider!
ばかでかいハンバーガーを食べた。
I ate a giant hamburger.