"Our horse is big."

Translation: Nasz koń jest duży.

[Rpeluffo]

What is the difference between Nasz and Wasz?

[immery]

nasz- our (belonging to me and at least one other person)

wasz - your ( belonging to you RodrigoPel16 and at least one other person)

[OleTok]

Macie slowo "wielky"?

[Jellei]

"wielki", tak.

But it's more than just "big".

[Hugo647250]

"Great" also in reputational sense, like Kasimierz Welki?

[Okcydent]

Yes. Kazimierz Wielki, Aleksander Wielki (Alexander the Great) and so on.

Wielki: more than simply big; of great insensitivity; very important; about man outstanding

Also:
Big Bang → Wielki Wybuch
Holy Week → Wielki Tydzień
Lent → Wielki Post
mass extinction → wielkie wymieranie (masowe wymieranie)