Expanding the size of a Raw Device Mapping (RDM)

Get the Physical LUN vml.xxx from the settings of the hard disk on the VM in vCenter/ESXi Host UI.
And then use the following commands on an ESX host connected to the storage:

# Get what naa.xxx and what is LUN number.
esxcfg-mpath -b -d vml.xxx

# More LUN info.
esxcfg-scsidevs -l -d vml.xxx 

Source: Binoche on VMware Communities - Solved: help with " Physical LUN and datastore mapping File "