How long were the Lost 116 Pages?
Valid configurations of the plates require sufficient surface area to fit the entire text of the Book of Mormon, as well as the text of the lost 116 pages. However, even though the lost manuscript is referred to as the "lost 116 pages," the specific amount of text from the plates that this represents is unknown.
Examining the references made to this lost text in the Book of Mormon and Doctrine and Covenants seem to indicate that the lost 116 pages were less detailed, implying they had less characters than the small plates of Nephi. The following is an exposition starting with Doctrine and Covenants 10, which references the lost 116 pages:
Doctrine and Covenants 10:38 And now, verily I say unto you, that an account of those things that you have written, which have gone out of your hands, is engraven upon the plates of Nephi;
[An "account" of the 116 pages is on the "plates of Nephi."]
The next verse characterizes this "account":
Doctrine and Covenants 10:39 Yea, and you remember it was said in those writings that a more particular account was given of these things upon the plates of Nephi.
[A "more particular" account of the lost 116 pages is on the "plates of Nephi."]
The Book of Mormon appears to use the term "particular" to mean larger or more detailed:
1 Nephi 19:2 And I knew not at the time when I made them that I should be commanded of the Lord to make these plates; wherefore, the record of my father, and the genealogy of his fathers, and the more part of all our proceedings in the wilderness are engraven upon those first plates of which I have spoken; wherefore, the things which transpired before I made these plates are, of a truth, more particularly made mention upon the first plates.
[The large plates of Nephi are a more "particular" version of the small plates.]
2 Nephi 5:33 And if my people desire to know the more particular part of the history of my people they must search mine other plates.
[If people want to know more "particular" history, they should refer to the large plates.]
Helaman 3:13 And now there are many records kept of the proceedings of this people, by many of this people, which are particular and very large, concerning them.
[There are many records that are "particular" and very large.]
So, a "more particular" account, or a more detailed account, or an account with more information, is on the "plates of Nephi." i.e., The "plates of Nephi" has more information than the lost 116 pages.
Verse 41 of Section 10 gives more detail about what is meant by the "plates of Nephi":
Doctrine and Covenants 10:41 Therefore, you shall translate the engravings which are on the plates of Nephi, down even till you come to the reign of king Benjamin, or until you come to that which you have translated, which you have retained;
[So you, Joseph Smith, shall translate the "plates of Nephi" which include the reign of King Benjamin.]
Words of Mormon indicates that plates that contain the reign of King Benjamin are the ones he is adding to record:
Words of Mormon 1:3-4 . . . I searched among the records which had been delivered into my hands, and I found these plates, which contained this small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi. And the things which are upon these plates pleasing me, because of the prophecies of the coming of Christ;
[I, Mormon, found "these plates" that are a "small account" with Nephi and Jacob down to King Benjamin.]
Words of Mormon 1:6 But behold, I shall take these plates, which contain these prophesyings and revelations, and put them with the remainder of my record...
[I, Mormon, am adding these plates that include Jacob, Nephi, and the reign of King Benjamin, to the Book of Mormon.]
The "plates of Nephi" are plates that contained the reign of King Benjamin, Jacob, and Nephi, which Mormon added to his own plates, which Joseph Smith had in his possession that he could translate. In other words, the "plates of Nephi" were the small plates of Nephi.
Based on the references to the lost text in Doctrine and Covenants 10 and Words of Mormon, it seems reasonable to conclude that the lost 116 pages are a less detailed, smaller account of the small plates of Nephi.
In terms of character count, the small plates of Nephi (1 Nephi through Omni) are about 360,000 characters.