Is this genetive? Does "potrzebuję" take the genitive?
yes. it is genitive, yes potrzebuję needs object in genitive.
Why not "I require a big bottle"?
I think it really translates to "wymagam" and sounds rather strange.
Why is it not dużej=bigger?
The -ej ending is because it is genetive. I need a bigger bottle = Potrzebuję większej butelki (I think!) Duży/dużej= big większy/większej=bigger
Thanks, it's clear now. I must find something good about Polish grammar because learning words is easy but I still don't know rules of conjugations and declensions.
Z powodu wirusa Corona muszę zostać w domu. Potrzebuję kilku dużych butelek.
Sit down please.