The SteelFusion Core may automatically detect the change in LUN size depending on how busy the LUN is. If there are read/write operations from the backup with a "LUN configuration changed" status, the Core will be forced to rescan and retry and it will automatically change. There’s also a periodic rescan every 5 minutes that would change it. You can also do a manual rescan and force the change. The size will highlight in orange when it changes.
Once the Core sees the change, the new size will be propagated to the Edge. If the LUN was pinned, the Edge will approve or deny the size increase based on Blockstore capacity.
You can read more in my blog post here: http://www.shocknetwork.com/wordpress/2016/01/13/how-to-expand-a-lun-with-steelfusion/