Part of this, of course, depended on how much room he had: how long was the work itself? By his own calculations on that sheet,* Tolkien worked it out to roughly 89,000 words.
Now, this is far less than he told Stanley Unwin during the period when he was trying to get Unwin to reject the book so Tolkien cd take it to Milton Waldman instead. In February 1950 Tolkien tells Unwin the newly finished book is 600,000 words (S&H Chronology .358). A month later this tally has risen to a million words (S&H.358), and by April to 'one million, two hundred thousand words' (.361); the hapless Unwin was pricing out an edition of '2,500 copies in two large volumes, each of 1392 pages' (.359).
So we know how Tolkien derived the figure of 89,000 words: the question becomes, how did he come up with the 600 thousand and then 1.2 million?
The 600k/1.2 million is the easier of these two questions to answer. Tolkien had told Naomi Mitchison in mid-December 1949 that he hoped to soon see in print 'two long books' (.354): this is clearly (1) LotR and (2) Silm. Since Tolkien was insisting the works be treated as two volumes of a single work, Unwin seems to have taken him at his word and priced it out accordingly, with each volume being 600,000 words in length.
The harder question, for me, is how Tolkien got from his estimate of 89,000 words, probably arrived at shortly after he completed the typescript (Oct 1949; C&H.352), to a claim just three months later that it was more than six times that length. The answer's does not hinge on the appendices, since both tallies omit them: Tolkien is clear in his Feb. 1950 letter to Unwin that the 600,000 total is the book's length 'even without certain necessary adjuncts' (.358) -- i.e., the appendices material. I hate to say it, but I suspect Tolkien blew up the total in order to discourage Allen & Unwin, not realizing how tenacious they wd be in hopes of working out an acceptable compromise.
Which raises the question: how long is the book, really? If I remember rightly, Lin Carter pegged it at a quarter of a million words, but I have not had time to go back through his little book and confirm or correct my memory on this point. Various internet sites each offer a different total:
The LotR Project: 481,103 words
—cf. 95.356 for THE HOBBIT and 130.115 for SILM
—this tally omits the appendices "and stuff not directly belonging to the text itself."
—this total puts it ahead of GONE WITH THE WIND (418.053) but still behind WAR & PEACE (587.287) and way far ahead of THE LION THE WITCH & THE WARDROBE (36.363) and WATERSHIP DOWN (156.154)
Better Storytelling: 455.125
—this article gives only the totals for the three individual volumes, which I've added together to get the total shown.
Fantasy Faction: 473k
So there is no reliable exact figure, but the consensus seems to be about a half-million words.
That's one big book, yet still significantly shorter (by about 25%) than Tolkien told the Unwins.
As for those 'certain necessary adjuncts', those will be the subject of the next post.
current reading: THE WELL AT THE WORLD'S END
*927 typed pages, 48 lines per page, twelve words per line = 88, 992 words (approx.)