We read poems which are quite lengthy which were written before novels were written, and poems which are primarily statements of poetics, novels, plays, etc.,
long poems with models which are long works (poems incl. epics, plays, works of fiction, non fiction, and/or novels),
long poems with sources which are long works (poems, epics, plays, works of non fiction, novels, and/or entire bodies of work by another author).
We read poems inspired by a false vacuum (lack of women's epics).
I think it is right to introduce the POstMOdern long poem when talking about long poems (I like "big poem" since it seems to me that some shorter poems are big), lyric series, and narrative long poem, because I think it is right to include the varieties of contemporary practice in our conversation.
We read poems which achieve length (or amplitude) in various ways: lyric "linking," semiotic development, dialogue, narrative/plot/voice/story, collage, and the like.
In one personal example, my long book (2d&i) was reviewed as including (long poems in and of themselves) poems written as the apparatus of a book (concordance, index, toc) as "a joke about getting beyond the page length minimum," but that's not why I wrote those poems. The new series by the same publisher amps up the length to --- I think? can't find the letter of invitation -- books of more than 1000 pages? In order to exclude books like my much shorter 300+ page book, which I actually personally wrote, in favor of machine-written (once programmed) or improvised variations and translations.
In another example, I wrote some of the only flarfy long poems (six of them) in my critique of flarf and first of my Kitty books, Secret Kitty (free at Ahadada Books, and I am an acquisitions editor there, so...), BECAUSE for no apparent reason poems written using search engines did not take into account 1) what was indexed, and 2) reinscribed lyric-narrative practice (including paucity of long works).