What are the longest texts, say top 5, transmitted via manuscript from the Classical/Early Medieval period?
The longest in one package may be
Codex Amiatinus, https://smarthistory.org/codex-amiatinus/ which is the earliest complete text of the Bible.
In monastery Libraries the commonest large multivolume work with most pages was Gregory's Commentary on Job, but this is partly because it was dinnertime reading in big script designed to be read by candle-light.
The longest classical texts change abruptly with the move from the scroll to the codex. Just as there is a jump from manuscript to printed book.
Among the printed books:
Cicero Orationes in 12 Volumes 1642;
Ovid 3 volumes.1682
Euclid in fifteen books (translated, I think, by Boethius c.500 Gk to Latin (Lost) and by Adelard of Bath c1100 Arabic to Latin.)
Vergilius Aeneid in six books.
Lucretius DRM in six books.
Cerberus made this valid point:
Can you define length? Number of pages in modern print? In Ancient manuscript? In number of sentences? Words? Letters? I think in words or letters will be quite difficult to compare...
In fact, if we were to merely define length by size, then the longest text would be the Codex Gigas (13th century though), which is the largest manuscript of the Bible ever made (before the printing press) (it also contains other texts though; see article). Here is a picture, just to given a idea of its size:
There are other examples of massive medieval bibles (e.g. here).
Although we only have parts of it, this book, written in the 5c A.D., is one of the longest extant works from Late Antiquity.
I also find it strange you'd discount the Vulgate, the work (while composed of smaller works), was a singular translation effort by one individual. In essence, it is a singular, continuous text by one author with a ballpark of 600,000+ words.
In response to the comments:
Jerome did start mostly from scratch and didn't just use the Vetus Latina. I'm currently making a new version of the Vulgate. The writing style is very consistent throughout, the only obvious differences are between the old and new testaments (probably a side effect of translating Hebrew vs Greek). He went back to the Hebrew sources and didn't just translate the Septuagint (which is what the Vetus Latina did). To me, the evidence that he at least did revisions throughout is shown by word choice and grammatical constructions. Uses of specific spellings like inclytus (inclitus), obēd- vs oboed- haere- vs hērē- are consistent throughout the work. Also, word preferences are shown throughout, specifically, Jerome loves cūnctus -a -um and uses it at every opportunity.
As far as books of the Vetus Latina included, these seem to be the Apocrypha (if wikipedia is correct as you put it), Maccabees, Sapientia, Ecclesiasticus, etc. I would have to look at Vetus Latina sections of works outside of the Gospels to determine if they are identical or not. I do know he translated the old testament from the Hebrew, and that alone is probably the longest single text in Latin. The entire new testament represents less than 20% of the bible. So if we're splitting hairs about whether he redid the entire new testament or not, it doesn't really matter, we still have 80% of the entire bible done mostly (if not entirely) by him.
Also, post Jerome additions to the Vulgate can't really be put on him as plagiarism, but nonetheless, the Vulgate stands as the longest Latin work to date (even if it is post-classical, but not really medieval either).