From what you told, there must be somewhere on your server a file that you use as attachment.
Dim filePath As String = AppSettings.Get("CMSDIR") & AppSettings.Get("Local_DIR") & AppSettings.Get("RptFileName") & AppSettings.Get("xls_extension")
Dim fi As FileInfo = new FileInfo(filePath);
If (fi.Length > 2097152)
[your logic here when you want to send it to ftp]
[your logic here when you want as attachment]
Hope this helps.