David Whitmer is recorded as teaching that the plates were 8 inches square, were as thick as tin, and had a whiteish yellow color.
E. D. Howe, Mormonism Unvailed (Painesville: E. D. Howe, 1834), 16
[David Whitmer] describes the plates as being about eight inches square, the leaves being metal of a whitish yellow color, and of the thickness of tin plates. The back was secured with three small rings of the same metal, passing through each leaf in succession; that the leaves were divided equidistant between the back and the edge, by cutting the plates in two parts, and again united with solder, so that the front might be opened, while the back part remained stationary and immovable, and was consequently a sealed book, which would not be revealed for ages to come, and which Sith himself was not permitted to understand. On opening that part of the book which was not secured by seals, he discovered inscribed on the aforesaid plates, divers and wonderful characters, some large and some small, but beyond the wisdom of man to understand without supernatural aid this account is sometimes partly contradicted by Harris.