Btrfs can pool drives of different sizes/types/whatever. You don't have to even do anything, you just say like: here is a new drive, add it to /home or whatever.
Obviously if you're actually using RAID for the redundancy or for striping then this is of no use. Oh it seems to do that too.
Pertinent info:
https://wiki.archlinux.org/index.php/Btrfs#Multi-device_filesystem_and_RAID_featureEDITED: 11 Apr 2014 01:37 by X3N0PH0N