From the Star Trek Giant Poster Book #10, June 1977, article "The Smithsonian Report":
The Klingon battle cruiser is 31" L, 20" W, and 7.5" H. In cm that's about 79 L, 51 W, 19 H.
From David Shaw info, the 3-footer was just under 34" long, and 15" wide.
Ah!

I still have most of the poster books, and they were a good source of then-updated information in the years following the Making of Star Trek / The World of Star Trek / The Trouble with Tribbles books. Just the kind of cant-miss publication for a 70s TOS fan.