I realise I've come to this discussion a little late, but here's my tuppence-worth.
I have always understood that as a rule of thumb, you should assume about 250 words to a page.
So every 100 pages would be about 25k words; 75k words would be about 300 pages.
100k words would therefore be about 450 pages, which is about as much as I'd want to read in a single volume.
But bear in mind that most ebook formats are compressed (RTF is not compressed), so if you have mostly text and few illustrations, most novel-length books should turn out to be under 1MB.