how long should a preface be?

What do you mean by preface? That is not the same thing as a prologue, just so you are aware. Also, a preface isn't necessary to a book and it seems to me like you are asking as if every MG novel has one in it - they don't, just in case that was something you were concerned about.

Anyway, yeah, a bit more information would be useful. But as always the answer is likely to be: as long as the story needs it to be.

Oh, okay, cool! Glad I could help! I mean, you can have a preface if you want one . . . I just wanted to make sure you understood what a preface was vs prologue, and that what you were asking after was the right thing. But somehow I seem to have cleared something up for you, so that's awesome!