Oddly enough, I really like series - and am more than willing to read 3-4-5 books in a series in a row, at 300-600 pages each, but tackling a 1000+ page single novel fills me with dread and I just really don't
like them.
I think it's something to do with being a goal-oriented person, wanting to reach the end (or a designated end point of an instalment in a series) within a "reasonable" (to me) time frame. If I don't manage to finish a book in 4-5 days, I start to get antsy and lose interest.
And yes, I know it doesn't make a lot of sense logically, but when have reading preferences ever made sense anyway?
That said, if it's a new book in a series I adore, I tend to hope it's longer and don't have issues with them being up to ~800 pages.