"A big house with small windows"
Translation:Duży dom z małymi oknami
I believe you meant 'z' :)
English doesn't distinguish between the 'comitative with' (in the presence of, accompanied by, containing) and the 'instrumental with' (by means of, using). Use z + instrumental case for the comitative meaning and just the instrumental case without a preposition for the instrumental meaning.
the whole phrase is in nominative- as if it were a subject of a sentence. "Duży dom " is in Nominative, but you need instrumental case after "z" in this meaning, so małe okna becomes małymi oknami
If the phrese was a paret of sentence, that needs to be in genitive- like "I am looking for a big house with small windows"- a sentence would be "Szukam dużego domu z małymi oknami".