I want to reduce the size of the internal hard drive of the emulator.

No, Genymotion Desktop does not provide a supported way to shrink the internal /data storage of an emulator. The only storage-related feature documented here is external storage for Genymotion Cloud, and it is not available with Genymotion Desktop. External Storage Virtual Devices Settings

Are you trying to free space on an existing virtual device or set up a smaller one from the start?