Exmerge from Exchange 2003 can import only up to 1.96 GB, its limitation on Exchange 2003 Exmerge.
Other options
– Try to export mail from outlook to a file and point it to PST
– Export using Exmerge by giving option to export with specific dates, bit tedious but possible
– User Exchange powershell Export-Mailbox command to export mailbox bigger than 2 Gb.
syntax: Export-mailbox -Identity “targetemailaddress” -PSTFolderPath “Targetpstpath”