Harris gives the weight of plates as between 40-60 pounds.
Martin Harris, Interview; in Iowa State Register, 28 August 1870, 1
A few days since we acknowledged a call at our sanctum, from Martin Harris, who was on his way from Ohio to take up his residence at Salt lake City, to spend the remainder of his days with the "chosen people." Mr. Harris was now in his 88th year, though still quite vigorous and sprightly, and he is Mormon, soul and body. He, as he claims, and as Mormons claim, together with two others, Oliver Cowdery deceased, and David Whitmer, now an apostate living in Missouri, were the divinely appointed witnesses to the Book of Mormon. The old gentleman evidently loves to relate the incidents with which he was personally connected, and he does it with wonderful enthusiasm.
In September, 1828, as the story goes, Joseph Smith, directed by an angel, proceeded to a spot about 4 miles from Palmyra, New York, and upon the point of a hill, extending northward, dug up a very solid stone chest within which were the tablets of gold, inscribed with the characters which no man could read. Joseph Smith was the first to handle the tablets, and Martin Harris, one of the appointed witnesses, the second. Mr. Harris describes the plates as being of thin leaves of gold, measuring seven by eight inches, and weighing altogether, from forty to sixty lbs. There was also found in the chest, the urim and thummim, by means of which the writing upon the plates was translated, but not until after the most learned men had exhausted their knowledge of letters in the vain effort to decipher the characters.
It had been revealed to Joseph Smith that the writing upon the tablets contained a history of the aborigines of this country down to the time of Columbus' discovery, and after, all human means had failed to secure a translation, Smith was commissioned to undertake the task. By means of the urim and thummim "a pair of large spectacles," as Mr. Harris termed them, the translation was made, and Mr. Harris claims to have written, of the translations as they were given by Smith, "116 solid pages of cap." The remainder was written by others.