Are the chaptors in the published book going to be the same length and those that are published on ao3? Or will some be meshed together? (Sorry, I only became a fan after the first two were already published, I don't know if this should be obvious or not l

No that’s a totally valid question :)

The parts that I posted prior to today were effectively chapters 1 and 2 of the actual book length chapters.

Then again, because the book is in its early editing stages that might also change.