Please stop disliking questions. Before I gave it a like it had a -1. This site is supposed to be a place to learn and therefore everyone has the right to ask questions about things they don't understand.
It's genitive because it's a sentence that states a specified/unspecified quantity of something. ex:) Butelka wody, butelka mleka, kilo kiełbasy (kilo of sausage), kieliszek wina (a glass of wine), 60 gramów cukru (60 grams of sugar), etc.
Basically, any "X of Y" phrase will take the genitive case.
"A small water bottle" in Polish is "mała wodna* butelka" ;)
adjective of woda
How would you use that phrase in a sentence? (preferably in a sentence that you can use in everyday life)
Well, with Nominative as here, I guess mostly simple "This is a small bottle of water"...
If you wanted to buy one, you could use Accusative and say "Poproszę małą butelkę wody" (A small bottle of water, please"