Longest novel for me is The Stand although its really three novels in one, Pandemic, Coming Together and Confrontation. That was 1200ish pages paperback.
I tried three times Noble House by James Clavell I loved Tai Pan but never made it past 400 on Noble House.
I read quite a few in the 80s the 1000 page epics by James Michener. Seriously read The Source and Chesapeake fantastic novels. One of the few writers who can make a location a character. Robert Parker also was brilliant at this turning Boston into a character. I loooooooooved his Spenser novels it should have ended with Hundred Dollar Baby.