Disk Provisioned Size Greyed Out

I have a virtual SCSI drive for a Windows 2008 64-bit guest.  I have recently run out of disk size and need to increase it.  From the online help, apparantely I can increase the size even while the VM is running.  However, whether running, suspended, or shutdown, I am not able to increase the disk size.  The option is greyed out, as you can see below.  What am I missing?

Thanks!