Maximum Message Size in Outlook Web Access in Exchange 2007

 
If you receive a message on top of your browser:
The files <file names> were not attached because they exceed the maximum size limit of <size limit> megabytes (MB) for attachments.

Configure message size limits for you organization.

1. Start the Exchange Management Shell.

2. Type the following command:
   Set-TransportConfig -MaxSendSize Size <value>

   For example, to set the maximum message size to 10 MB, type:
   Set-TransportConfig -MaxSendSize 10MB.

3. Restart Microsoft Exchange Information Store service to apply change.

To configure Outlook Web Access to allow content that is equal to the size of the maximum message size on a Windows Server 2008-based computer

1. At a command prompt, type the following:
   cd %windir%\system32\inetsrv

2. Type the following command:
   appcmd set config "Default Web Site/owa" -section:requestFiltering -requestLimits.maxAllowedContentLength:<Content_Length_In_Bytes>

See:
http://technet.microsoft.com/en-us/library/aa996835(EXCHG.80).aspx
For Exchange 2010:
http://technet.microsoft.com/en-us/library/aa996835.aspx