Raindrops the size of bullets thundered on the castle windows for days on end; the lake rose, the flowerbeds turned into muddy streams and Hagrid's pumpkins swelled to the size of garden sheds.
My question is how we should take "Raindrops the size of bullets" from the grammar standpoint. Is it a noun phrase "the size of bullets" modifying another noun word 'Raindrops'? Or it's kind of omission for Raindrops which had the size of bullets?
How should we understand it from the grammar standpoint?
-- From Harry Potter.