I was tasked to update Exchange 2007 message size limits.  For the casual Exchange admin, this can be a rather daunting task. Here are a couple links I found that were helpful.

http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=1545320&SiteID=17

http://technet.microsoft.com/en-us/library/bb124345(EXCHG.80).aspx

Steve