Lemme go get my tape measure...
8 and 3/8 inches square.
Just so you know, I checked Ebay and Igor and KEH and about everyone else who deals in used large format gear I could find, so...yeah.
...except the Large Format Photography forum. *headdesk*
If yours won't fit, I'll see if I can still login over there and ask the guys. If I can't find an original back, I'm going to build one.