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........