Something like this is best answered by someone who works at an Esperanto library. I am not sure, but I can give you a list of contenders. I don't own these books, so I can only judge by the reported number of pages.
The standard translation of the Bible has a total of 2255 pages, 1760+495, in the Inko edition (which uses the normal format of a paperback book). The book ADORU: Ekumena Diserva Libro (2001) has 1492 pages.
The four classic Chinese novels are similar in length, but it looks as though the translation of Pilgrimo al la Okcidento holds the record for a translation of one work by one person, with 1976 pages.
Translations
From Chinese:
- Ruĝdoma sonĝo (tr. Xie Yuming, 1997): 1893
- Ĉe akvorando (tr. Laŭlum, 2004): 1830
- Romano pri la Tri Regnoj (tr. Laŭlum, 2008): 1807
- Pilgrimo al la Okcidento (tr. Laŭlum, 2009): 1976
From other languages:
- La mastro de l' ringoj (tr. William Auld, 1997): 1158
- La grafo de Montekristo (tr. Daniel Moirand, 2010): 1184
Original novels
Longest novel in Esperanto (by reported number of pages), in succession:
- Kastelo de Prelongo (Henri Vallienne, 1907): 515
- Idoj de Orfeo (H.J. Bulthuis, 1923): 542
- La granda kaldrono (John Islay Francis, 1978): 592
- Tarokoj kaj epokoj (Christian Declerck, 2003): 656
There are two other novels with more than 600 pages, La princo ĉe la hunoj (Eugène de Zilah, 2011) and Morto de artisto (Anna Löwenstein, 2008). I have to point out however that Kastelo de Prelongo has 700 pages in the Inko edition, so these lists do not give a reliable ranking order.
For translated non-fiction, there is La kapitalo, unua volumo (Wilhelm Luttermann, 2011): 762 pages. For poetry, there is Testamente: plena originala poemaro (Edwin De Kock, 2015): 698 pages.