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.
Wait... I thought jest required instrumental case. What am I forgetting...
In this sentence, you have adjective "alone". In this type of sentence adjective is in nominative.
Ten piest jest duży. (this dog is big)
On jest dużym psem. (he/it is a big dog)
Ten pies just dizzy. God, the hardest thing about learning polish so far is to not see English words!