I found the answer in HamGadgets' FAQ:
http://www.hamgadgets.com/index.php?main_page=page&id=7#id-o-matic.
The board measures 1.95" x 2.5" (49.5mm x 63.5mm). Hope this helps someone else who may have a spare box laying around the house. I now know that mine project box will work quite well!
73,
Michael