Addison
Auror
The number of books in a series, although currently trending at three, doesn't matter to anyone or thing but you and your story. If it looks like it will take four books, then it will take four books. As for length, again it depends on the story. The Harry Potter series grew in length from 1-4 but 5, 6, and 7 ranged. True that Deathly Hallows was the longest but for obvious reasons. But six was shorter than five. There's no right or wrong answer when it comes to length in any form for writing...unless it's a short story then you're stuck. 
