title: Display two object same real distance (e.g. inches) apart across different browers / screen sizes
body: <p>I'm developing a psychology experiment in the browser. In order to keep the same viewing angle across people, I want to display two characters around 5 inches apart on the screen. </p>
  
  <p>Is there any way to detect the real size of the monitor being used, and using the screen resolution and DPI, render the two objects the same <em>real</em> width apart? (I will only allow people that have real computers, e.g. not mobile)</p>
  
  <p>I heard detecting real size may not be possible, if true, and assuming people will report to me the real size of their monitor, is this possible?</p>
  
  <p>I'm using HTML5 Canvas, fwiw. Perhaps resizing this canvas w.r.t to the resolution and DPI is a solution.</p>