vSphere 4 and newer allow growing datastores on shared storage on the fly (i.e. online). First expand the LUN, rescan then grow the datastore, which is actually nothing more than resizing the existing VMFS partition.
Please be aware of the maximum LUN size of of 2TB minus 512Bytes for datastores formatted with VMFS3. In order to use larger LUNs, you need to have all hosts which access the datastore on ESXi 5.x and the datastore has to be VMFS5.
André