I didn't install the latest VSA version 5.1 myself yet, but I certainly would not expect this question. Can you confirm it's version 5.1 you are using and that there was no option to select to use only a certain amount of disk space? (see http://blogs.vmware.com/vsphere/2012/08/new-features-of-the-vsphere-storage-appliance-version-5-1.html)
André