Hello.
You have a few options.
You can adjust the batch size of the File Receive Adapter. Or, you can adjust the High / Low water marks associated with Messages. This blog posts talks about it a little more: http://blogs.msdn.com/darrenj/archive/2005/04/17/409068.aspx
This Performance Document might help you out: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/BTS_2004WP/html/04d20926-20d2-4098-b701-52238a267eba.asp