'dozen', like "cup" (e.g., a cup of beans) refers to a measurable quantity, so 'is' would be the appropriate verb. There's also a hint there: Note, the determiner 'A' is also singular:
A dozen is enough. ('a dozen' and 'is' agree in number)
Plural 'are' is also used by native speakers, especially if they consider 'roses' to be the subject:
A dozen roses are enough. ('roses' and 'are' agree in number)
'A dozen roses are' is not the Standard, but it's what some speaker might use because the word 'roses' is closer to the verb.
All the best,