You cannot decrease the disk size for Windows-based and FreeBSD-based virtual servers. Only the increase disk size option is available.

This was done intentionally to protect against file corruption and data loss within the OnApp environment.

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.