A prolix piece of writing has too many words and is boring. It is not merely long; it is long in a weary way — the writer will not come to the point. You meet the word in book reviews, in teachers’ comments, and in rather formal English. Ordinary people just say too wordy, or it drags. There is a verb, from the same adjective, meaning to go on at that tedious length, but it is rare; the adjective is the form you will actually see. Though actually, a careful writer can be long without being prolix: a long story that earns every page is not prolix, and a short memo that still wastes words can be.