A standard Windows 7 install can take up 16GB+ of space. "Lite" or "Super-Nano" versions are often trimmed down to under 3GB .
The QCOW2 (QEMU Copy-On-Write) format is ideal because it doesn't allocate all disk space at once, making it easier to clone and snapshot. windows 7 lite qcow2 best
While Windows 7 naturally runs on older hardware, "Lite" versions can function with as little as 512MB of RAM . Top Sources for Windows 7 Lite Images A standard Windows 7 install can take up 16GB+ of space