Controlling attachment size in Exchange Server 2007 Outlook Web Access (OWA)
In this MSExchangeTeam post we'll understand how to configure OWA to allow larger messages.
By default, Exchange 2007 OWA will let you send attachments of up to 30 MB in size. Depending on your situation, you might want to either increase or decrease this number as we have a mechanism by which you can change the default upload limits of file attachments in OWA.
Read more at source: http://msexchangeteam.com/archive/2006/11/14/431477.aspx