Remote mobile testing

I need to do some remote mobile user testing of prototypes in a few weeks. What would be the best way to do this?

I checked out the app but that requires the user to install and then log in and gives them access they shouldn’t have. So definitely a no

I tried out just a 360 X 640 rectangle on apage and loaded up the page directly in the browser and it worked though was tiny on my screen.

Ideally I want the prototype to appear as full-screen as possible and I don’t know what resolution user’s phones will be as they are using their own phones. Is there a better way than the second option?