I don't claim to be an expert but I believe most books go through some form of editing before publication so, even if you did write 'War and Peace' it may well be edited down to something more reasonable. That is why JKR's first books were shorter. They were edited down. Once she really took off the publisher more or less let her write what she wanted, which is why there is so much of irrelevance to the plot in her later books.
Its a bit labourious but some simple maths should do the trick.
Depending on point size there are around 15 words per line.
Again depending on point size thereare around 35 to 50 lines per page and then just times it by the page numbers.
Bit rough but you get the idea.