Unfortunately it is impossible to resize Windows vms in OnApp 3.0.x this was done intentionally to prevent disk damage and data loss that was occurring when people were resizing before.
Unfortunately, it was built into OnApp that it will not allow disk decreases on Windows vms.
The only option available is to take a snapshot of the server, convert it to a template, and try to build using a smaller disk, which may or may not work correctly, or to deploy a new server, and migrate your content and settings.