Of course, width of the board is a significant factor in warpage but  you 
also need to consider the following factors:

1.   Board thickness
2.   Array design (spacing, break-away locations)
3.   Material of the board
4.   Even distribution of copper layer
5.   Number of layer (EVEN prefer)
6.   Copper thickness (should be symetric)
7.   Dielectric thickness (should be symetric)
8.   Web design (Copper filled??)

Thanks
Michael Yuen

 ----------
From: TechNet-request
To: TechNet
Subject: Assy: max board width
Date: Thursday, June 13, 1996 11:40AM

     We are considering changing our board size and are curious about how
     wide the boards can be to avoid warping in reflow. We plan on doing 1
     and 2 reflows (mostly just one reflow) and we do not have support in
     the middle.