On a Container Linux machine, the operating system itself is mounted as a read-only partition at /usr. The root partition provides read-write storage by default and on a fresh install is mostly blank. The default size of this partition depends on the platform but it is usually between 3GB and 16GB. If more space is required simply extend the virtual machine's disk image and Container Linux will fix the partition table and resize the root partition to fill the disk on the next boot.
Amazon doesn't support directly resizing volumes, you must take a snapshot and create a new volume based on that snapshot. Refer to the AWS EC2 documentation on expanding EBS volumes for detailed instructions.
Even if you are not using Qemu itself the qemu-img tool is the easiest to use. It will work on raw, qcow2, vmdk, and most other formats. The command accepts either an absolute size or a relative size by by adding + prefix. Unit suffixes such as G or M are also supported.
# Increase the disk size by 5GB
qemu-img resize coreos_production_qemu_image.img +5GThe interface available for resizing disks in VMware varies depending on the product. See this Knowledge Base article for details. Most products include a tool called vmware-vdiskmanager. The size must be the absolute disk size, relative sizes are not supported so be careful to only increase the size, not shrink it. The unit suffixes Gb and Mb are supported.
# Set the disk size to 20GB
vmware-vdiskmanager -x 20Gb coreos_developer_vmware_insecure.vmxUse qemu-img or vmware-vdiskmanager as described above. VirtualBox does not support resizing VMDK disk images, only VDI and VHD disks. Meanwhile VirtualBox only supports using VMDK disk images with the OVF config file format used for importing/exporting virtual machines.
If you have have no other options you can try converting the VMDK disk image to a VDI image and configuring a new virtual machine with it:
VBoxManage clonehd old.vmdk new.vdi --format VDI
VBoxManage modifyhd new.vdi --resize 20480