Here's an example, in which running this command increases the size of the virtual disk file "H:\VMs\W2K3\IDE00.vmdk" to 6 GB:
vmware-vdiskmanager.exe -x 6GB "H:\VMs\W2K3\IDE00.vmdk"
If you have not added the full path to vmware-vdiskmanager.exe to your Environmental Variables, you'll need to use the full path each time you runt the command.
For example:
"C:\Program Files\VMware\VMware Workstation\vmware-vdiskmanager.exe" -x 6GB "H:\VMs\W2K3\IDE00.vmdk"
After you've expanded the virtual disk size, you'll also need to expand the partition.
The easiest way to do this is with Acronis Disk Director.
Just set the .iso boot disk as the virtual CD Rom on your VM and use the utility to resize. It will take you less than 1 minute.