Google Assistant on Google Home, shockingly, does not include commands to help you find your phone by default, but with some help from IFTTT, it can be done. All it takes is a bit of setup and an IFTTT account, and you'll be able to simply say, "OK Google, find my phone", and hear it ring at full volume as though you had triggered the ringing through Android Device Manager. It is worth noting, though, that Do Not Disturb mode can keep Google Home from cranking up your ringer volume, if your phone happens to be running any Android version older than Android 7.0 (Nougat).
Thanks to a purpose-built IFTTT applet from Google, the steps to enable this feature are actually pretty simple. Get an IFTTT account, if you don't already have one, then login to the Google Assistant channel with your Google login. From there, activate the channel, as well as the Phone Call channel, on IFTTT. From there, hunt down the applet to have Google Assistant call your phone, which is on the Google Assistant channel. Just turn on the applet, then set a phrase or two to trigger it, what you want Google Assistant to say in response, and what you want the phone call to say when you pick it up. Save all that on the applet page, and from then on, your Google Home will be able to call your phone whenever you ask it to.
The second part of the setup involves the Android Device channel. There is no pre-created applet for this, so you'll have to create one, once you've activated the channel. Luckily, that's not all that hard. Simply create a new applet, and under This, go to the Google Assistant channel and set up trigger phrases identical to the ones you set up in the first step. Next, go to That, and pick the Android Device channel, where you will find "Set ringtone volume" under actions. Crank it to 100%, then hit Create Action, and finally, Finish. Once it's all said and done, uttering one of your trigger phrases around your Google Home will make your phone ring at max volume.