Grid sizes inconsistent depending on screens

This is also something I’ve been grappling with for some time now but I’m yet to understand the root of the problem. Is this purely a browser issue? Has anyone been able to find an approach to scaled layout which works consistently across different browsers and screens?
I’m concerned that I can’t always mimic what my clients and other external sources are seeing (ie occasionally text and graphics can be cut off at the top and bottom of screen) and I’d love to have Readymag highlight a text-free zone somewhere in the project settings.