Lapham reports that Smith Sr. gave dimensions of plates at 6x9-10 inches.
Fayette Lapham, Report of an Interview with Joseph Smith Sr., 1870; rep. Larry E. Morris, ed., A Documentary History of the Book of Mormon (New York, NY: Oxford University Press, 2019), 439
In answer to our question, as to what it was that Joseph had thus obtained, [Joseph Smith Sr.] said it consisted of a set of gold plates, about six inches wide, and nine or ten inches long. They were in the form of a book, half an inch thick, but were not bound at the back, like our books, but were held together by several gold rings, in such a way that the plates could be opened similar to a book.