Do NOT use Fireworks for print work. Use Illustrator, InkScape, InDesign, Scribus, Xara Designer Pro, Photoshop, etc, anything BUT Fireworks. Fireworks is meant for web graphics.
However, if you are still convinced it is a good idea to do this in Fw, there is a way to increase the max canvas size:
However, you can change the limit.
Make sure that Fireworks is closed, open
Fireworks CS6 Preferences.txt file located in:
C:\Documents and Settings\USERNAME\AppData\Roaming\Adobe\Fireworks CS6\English\
Library/Preferences/Adobe Fireworks CS6/en/
or adjust this number as needed.