when you need to say "big" in nominative - adjective before a noun in nominative ( subject, after "to") or noun/it is big. (without noun),
Duży is for masculine nouns - singular
Duża is for feminine nouns- singular
Duże is for neuter nouns - singular , and not masculine personal nouns -plural ( plural masculine not personal; feminine and neuter nouns)
Duzi is for masculine personal plural nouns.