disclaimer: this worked for me, your mileage may vary. Your Pi, your responsibility :)
After putting Arch Linux on a 16GB SD card using these instructions, I ended up with about 14GB of free space.
Arch Linux uses one primary partition (/dev/mmcblk0p1) and an extended partition (/dev/mmcblk0p2) containing one logical partition (/dev/mmcblk0p5). The primary partition is the boot partition and the logical partition is the root partition. Rather than adding another primary partition I just wanted to resize the root partition and filesystem.
According to this bugreport parted
no longer handles resizing of partitions and gparted needs a graphical environment to run. So I had to come up with something else to resize my partitions.