To increase a RDM, you need to increase that specific LUN on the storage, and not a datastore, although a .vmdk file - which is created for a RDM - is located on a datastore. This is only a mapping file pointing to the storage LUN and does not contain any production data.
If you are still using VMFS3 datastores, make sure that the VMFS block size is sufficient for the new LUN size before you increase it!
André