The current version that I have been working on this last week or so is about 7,000 words right now. I have an old version that is about 27,000 words but is based on some ideas about presentation and writing that are no longer being used (I have changed ideas about what audience I want to reach).
I don't feel very happy with my current version, either--I don't have a strong sense of structure for the ideas I want to express.
Nonetheless, I press on, because that is the only way that it will get any better. For now, I just need to stay with the project long enough to shape a complete draft of the appropriate size that is consistent and coherent within its own chosen purview.
So, really, my touchstone is the 7,000-word draft that I have right now.