OK, in my search for various facts and myths(?) about fanfics on this site, I've come to a reasonable general length of 2-2.5 pages per/chapter.
And that even seems like one of the shorter chapters.
I'll start writing chapter 1 soon, and then see how people likes it and if they can help me become better at writing. If there's people liking it, I'll continue it.
See it as a kinda "trial-chapter" of sorts.