qemu-img create -f qcow2 isotest.qcow2 10G
The above creates a "qcow2" format image called "isotest.qcow2" that is 10GB in size.
If you want to preallocate all of the space for increased performance do this:
qemu-img create -f qcow2 isotest.qcow2 -o preallocation=full 10G
It is controlled with the preallocation=full option, of course by default this is set to off (so not using -o preallocation really silently results in -o preallocation=off).
How to create from a template/backing image?
qemu-img create -f qcow2 newimagefile.qcow2 -o backing_file=yourfile.qcow2 10G
newimagefile.qcow2 is the new image you are created
yourfile.qcow2 is the base/template image that it is based off (eg. maybe you installed Windows or Linux to yourfile.qcow2 and you want to start from that point so the combined image will be the difference of the original "backing file" vs the changes you make which are reflected in newimagefile.qcow2 (so don't move or delete the original backing file yourfile.qcow2 or you will lose your data).