I prefer the word Establishment over, say, Introduction, although introduction is a key element. But in my mind, this first section of the book is about not only letting the reader know the main characters, who they are and what they are up to, but about getting a firm toehold into what the world is about. This is the foundation upon which the rest of the book is built.
Here we've got my main two characters, Katrine and Minox, with Katrine as the POV character, where she establishes herself in her new situation (see, there's that word) that sets the stage for what she's doing now. When I first drafted the outline for this book, I came up with the idea that Katrine had a big secret, and it would come out at part seven of the outline (Collapse). But there was no reasonable way to have Katrine's POV and dance around said secret, I realized. The secret, in and of itself, could not be the bomb that gets dropped. So I put it right in the beginning. Right in the very first sentence:
Katrine Casey walked to the Inemar Bank Constabulary House carrying a lie.
The over the course of the Establishment, we have her meet Minox, and get a grasp who he is, t least through her eyes. We also meet four other secondary characters, whose roles, I will admit, are not entirely fleshed out in the existing outline. That is part of the discovery of writing. The process of outlining never takes that away.
Anyhow, I've found that for the Establishment, around 5000-6000 words is a good zone, especially with a target length of 80-90K. It's also, for me, one of the harder parts to write, because it's part of the mental process of getting into the world and the characters. Once I've established that in my head, the rest becomes clearer.