There's a photo on the homepage: https://www.cerebras.net/ It's just about 8.5" across. You could just barely print a 1:1 photo of it on a letter-sized sheet of paper, with less than a half-milimeter margin on each side.
Yes, I it is probably a copper slab (they never mentioned), but there is no electrical connection, as the rest of the slides make clear: https://imgur.com/a/Rbd7e4D
It just provides a thermal connection for water cooling. The electrical connection is made through the PCB (and probably through a thick copper plate on the opposite side of the PCB)
