Pratt gives the dimensions of the plates as 7x8x6 inches.
Orson Pratt, A Interesting Account of Several Remarkable Visions, and of the Late Discovery of Ancient American Records (Edinburgh, Scotland: Ballantyne and Hughes, 1840), 12–13, The Joseph Smith Papers website, accessed August 11, 2021
These records were engraved on plates, which had the appearance of gold. Each plate was not far from seven by eight inches in width and length, being not quite as thick as common tin. They were filled on both sides with engravings, in Egyptian characters, and bound together in a volume, as the leaves of a book, and fastened at one edge with three rings running through the whole. This volume was something near six inches in thickness, a part of which was sealed. The characters or letters upon the unsealed part were small, and beautifully engraved. The whole book exhibited many marks of antiquity in its construction, as well as much skill in the art of engraving. With the records was found “a curious instrument, called by the ancients the Urim and Thummim, which consisted of two transparent stones, clear as crystal, set in the two rims of a bow. This was in use, in ancient times, by persons called seers. It was an instrument, by the use of which, they received revelation of things distant, or of things past or future.”