Whitmer gives dimensions of plates as 6x9 inches.
David Whitmer, Interview, Kansas City Journal, 5 June 1881; rep. "Mormonism," Millennial Star 43, no. 28 (July 11, 1881): 437
“Did you see the angel?”
“Yes, he stood before us. Our testimony as recorded in the Book of Mormon is absolutely true, just as it is written there.”
“Can you describe the plates?”
“They appeared to be of gold, about six by nine inches in size, about as thick as parchment, a great many in number, and bound together like the leaves of a book by massive rings passing through the back edges. The engravings upon them were very plain and of very curious appearance. Smith made fac similes of some of the plates and sent them by Martin Harris to Professors Anthon and Mitchell, of New York City, for examination. They pronounced the characters reformed Egyptian, but were unable to read them.”