Hi Julia,
Based on the test, we can add the following performance counter to monitor the file size of TempDB on a Windows Server 2008 that is running SQL Server 2008.
"SQL Server: Database (tempDB)\Data File(s) Size"
Please note: this counter won't appear until we install SQL server entry on the Windows Server 2008 because the counter is provided by SQL Server.
Hope it helps.
David Shen - MSFT