First of all, there is no way to do that in Figma. In Figma prototypes have static size, there is no way to make them responsive based on the device you are viewing them from. It’s possible in apps like Framer. Feel free to vote for this idea: Adaptive/Responsive Preview
Secondly I’ll mention that Figma Mirror isn’t made for sharing, you need to use your own account in it. If you want to share the prototype, you need to share it via a link of the prototype view.
No, nobody does this. You can use one size just fine. It will just have black areas on some screens with different the screen ratios.