Why would you want to power it off?
As others stated these Wi-Fi sockets almost use as much energy as the Pi.
Also, how would you power on the Pi after shutting down?
I don’t think this is a good idea either. This device should be powered on 24/7 so you can use it anytime you want. Powering ON/OFF can reduce the SD card life (during boot and post-boot it not only reads the SD but writes some log and other data to it), can make it much more difficult to use the MagicMirror, etc…
It is better if you control the screen remotely. Like sending a command to the Pi to ‘power off’ the HDMI port, so the screen should go to standy.
Anyway if you want a Wi-Fi plug, then I can recommend this one. It is small, has power meter, etc… However it is available for delivery in January.
However, I would love to see your ‘why’ would you want to do this. I have several RPi running in my house, each has it’s own purpose, but I wouldn’t turn off either of them… It just makes it more complicated, then what you can spare on this.