Also , check the following;
http://docs.oracle.com/cd/E26996_01/E18549/html/CHDIDBHI.htmlin these doc, it says;
To edit a virtual disk:
In the table of the Virtual Disks tab, select the virtual disk you wish to edit.
Click Edit Virtual Disk to open the Edit Virtual Disk dialog box. Enter a new name for the selected virtual disk, a new size in GB,
and optionally make it shareable. Click OK.
This doc is for VM Manager, so if you have VM MAnager in the envrionment, give it a try by doing the above..