The "version information" here lists it as:
"Avalon Hill Bookcase 11.50 x 8.40 x 2.10 inches or 29.2 x 21.3 x 5.3 cm"
Actually, this may be incorrect.
If you are talking about the bookcase games where the inner box slides into the outer cover (a la B&O/C&O, Outdoor Survival, etc), the dimensions are
8 9/16" x 11 3/4" x 2 1/4"
Not sure that would make a difference, but those slide in boxes are slightly
bigger than the Rail Baron, Acquire etc.. types