Appleby reports dimensions of the plates.
W. I. Appleby, A Dissertation of Nebuchadnezzar’s Dream (Philadelphia: Brown, Bicking & Guilbert, 1844), 23
Mr. Smith having been shown in the Vision the place where the Record was deposited; accordingly the next morning, 23d day of September, A. D. 1823, visited the place, and from that time he received more and more information concerning what he had to do until the year 1827, when he obtained the Recods [sic]. A hole of sufficient depth had been dug, and a flat stone laid in the bottom; then there were four set erect at the outer edges of the bottom stone, joined together with some kind of cement, so as to form a Box. On the bottom stone was laid a Shield or Breastplate, from that arose three pillars made of cement. On the top of these pillars laid the Record, together with the "Urim and Thummim," the whole not to extend quite even with the top of the side stones. Over the whole was placed a crowning stone, a small part of which was visible, when he first visited the spot. Each plate was about six by eight inches, and as thick as common tin, and on each side beautifully engraved, and filled with black cement, the whole being about six inches in thickness, and put together with three rings, running through the whole, a part of which was sealed. The unsealed part has been translated; and contains the Book of Mormon; and living witnesses, who saw the plates and hefted them have borne testimony to the same. In the year 1830, after the Book was translated and published, the Angel of God according to promise conferred the Holy Priesthood upon Mr. Smith, and gave unto him commandments to Baptize with water in the name of Jesus, and build up the Church of Christ; which accordingly was organized April 6th, A. D. 1830, with six members.