Having recently searched and found the solution for re-sizing your root filesystem size on a Raspberry pi under Arch Linux I discovered the following instructions to work:
Delete the second partition /dev/mmcblk0p2
Create a new primary partition and use default sizes prompted. This will then create a partiton that fills the disk
Save and exit fdisk:
Now reboot. Once rebooted:
Your main / partition should be the full size of the disk now.