There is a new multipathd command, "resize map <mapname>". This command is used to cause dm-multipath to adjust to a change in the size of the underlying block storage device.
After resizing the underlying block device, you can resize your multipath device by running:
# multipathd -k"resize map <mapname>"
# multipathd -k"resize map mpath0"
There is one restriction on the use of the device mapper multipath resize command. You can not resize a device-mapper device while there commands queued to that device. That is, do not use the resize command when no_path_retry is set to "queue", and there are no active paths to the device.