Well my first thought was about 15k words as a prequel. I reached that with ‘just the big battle scene to craft’ so I extended the expectation to 20k words. I just passed 20k this evening and still feel that over half the scene is still to come. Maybe 25-30k words in the first draft.
I will definately hack some (lots) of the dross out of the earlier parts, although probably not the very beginning, which I quite like.
Just got Hemingway software today. It claims to simplify and streamline your text. It spots sections that are difficult to understand, unneeded words, etc. looks like a useful piece of software. I ran the first draft very quickly through it this afternoon and was more than a little embarrassed at how much it thought was wrong. In my defence, this was just a very rough first draft.