I have mixed feelings on sequels. I simultaneously adore them and abhor them.
I love sequels because I love getting to know more about my beloved characters.
I hate sequels because I hate having to wait a year or more inbetween books, wondering what happens every second between books until I can’t concentr— Wait… What was I talking/writing about? (Not to mention cliffhangers! I can assure you, cliffhangers only increase my hatred of sequels, although I love cliffhangers.)
For my favorite series, I sometimes end up checking the latest release date every day of the week after I read it. Most often, my dreams/nightmares are also hounded by the characters. I often imagine throttling the authors because they don’t write faster.
In conclusion, I prefer large books so I can learn a satisfactory amount of my character’s story (yes, I refer to them as “my characters”) and no sequel, so I don’t have to wait. But, imagine the weight of the Harry Potter series as one book, so I suppose an author has to consider practicality when deciding whether or not to have one book or many.