Usually the x64 combined squashfs image will claim any space on the block device after the squashfs partition as overlay space, so it should be as large as the underlying medium.
How did you write the image exactly? Or did you maybe install the combined ext4 one by any chance? Because that one is only preformatted for an 100MB overlay partition. For that image you should extend the partition and run resize2fs after writing it to the blockdev.
Why not use ext4 instead, seems like the squashfs's just an additional pain,
and harder to resize.
Love the APUs, you can have multiple installations set up in parallell on the SDs,
and just swap by rebooting, and selecting whichever one you'd like to run.
As I understand it the squashfs installation is easier on flash file systems, but I don't really know. Honestly, I'm more or less just getting started on the APUs so I'm feeling my way around.