A while ago, I played around with a short story idea in a genre I don't normally write (Young Adult fiction). The story took on a life of its own, growing quickly to a novella, and now is definitely a novel, trying to stretch itself to a series.
The story itself is going great, I'm just not sure where/how to break it up for a YA audience. Specifically, I expect the content to be most appealing to the pre/young-teen crowd (12-14). So far (it's still not completely mapped out) there are seven 'parts' -- that is, major plot arcs secondary only to the main series-long arc.
Were I writing for adults, I'd combine parts 1-3 into Book I, parts 4-5 into Book II, and parts 6-7 into Book III. However, this might be too long for a YA audience, putting each book at 500+ pages with the longest at about 700 (guesstimated, of course, all but the first part are mostly notes).
It probably seems a bit early to try breaking things up, but one of my New Year's resolutions will be to make real time for my writing. To set goals, I'd like some idea of at which point in the plot Book I is "done".