Learn Croatian: Ova cvjećarnica ima najveće ruže. - This florist shop has the biggest roses.

It needs to agree with ruže in gender, number, and case. Because ruže is the direct object of the verb ima, it is in the accusative case. For feminine plural nouns, the accusative ends in -e. The matching feminine accusative plural form of the adjective najveći is najveće.

How is the superlative najveće built from the basic word for 'big'?

The adjective velik (big) has an irregular comparative form: veći (bigger). Like all superlatives in Croatian, you just add the prefix naj- to the comparative to make it 'the most'. So veći becomes najveći.

Since ruže can also be nominative plural, how do we know it's accusative in this sentence?

Feminine nouns ending in -a like ruža have the exact same ending (-e) for both nominative and accusative plural. You rely on the grammar of the sentence: ova cvjećarnica is the subject (nominative), and the verb ima requires a direct object, which must take the accusative case.

I often see words ending in -nica like cvjećarnica. Is this a standard pattern?