You will find that mapped RDM datastores in virtual compatibility mode cannot be extended simply by a re-scan of the HBA and extending the drive. In this case you will need to shut down the server, make note of the scsi x:y postion, un-map that volume from the virtual machine and re-map it again. This would recreate the descriptor file with the added space.
A simple rescan within your VM OS would show the added space which you can extended using DISKPART or any other tool out there.
RDM’s in physcial compatibility mode can be extended easily without the above steps.