First, create an empty image. A 64GB size is typical for lightweight installs, but it will only use the space Windows needs.
A raw img file for Windows 10 would immediately consume 30-60 GB of disk space. A qcow2 file starts small (usually 1-2 GB) and grows dynamically as Windows writes data. For example, a fresh Windows 10 installation might report 20 GB used inside the guest, but the host file may only be 12 GB due to compression and sparse allocation. windows 10qcow2