All of them? I don't think there's a way around that, including expecting the people writing your copy to operate in the abstract realm of responsive CSS or whatever.

There's no way around thinking about your use-cases and you're not going to convince designers to not want to see what the end product actually looks like.

I'm not sure I agree on your last point; some of the younger, less print-bound designers I've worked with have a good understanding of the difference between presentation and semantics, and have been often enough annoyed by sites which look just dandy at 1920x1080, but are unusable on a phone, to have a burning desire not to produce more of the same.

Stipulating that point, though, perhaps the solution is to keep designers at a higher level of granularity -- to have them produce designs for "large screen", "small screen", "mobile", "print", &c., including general ideas of how body text shall appear, but leave the more granular detail to someone who's not so much wedded to pixel-perfection, but instead understands that the same content will necessarily look different when differently rendered, and who can understand and accommodate those differences to produce a result that works well across all of them.

Then there is also retina vs non-retina displays, the next site redesign, glossy vs matte screens, and non-color calibrated displays. Then there are different browsers, operating systems, and fonts installed.

It is almost as funny as the request that a logo must use the "Pantone 1337" color. It probably is not representable in RGB and nobody has a color-calibrated screen.

Use the responsive design view built into firefox and try the different screen sizes. You still are going to care 'how it looks' on all of the devices you expect to see it. I understand that it's nearly impossible to control for every screen size, and maybe that's the point, but you should still know and understand what most users will see.

As long as the tool you're using doesn't allow people to inject arbitrary presentation instructions (eg. font size and style) along with the content you can safely allow people to enter their content on one device and know that your responsive style will handle the rest. We are solving this problem and looking for feedback at http://www.decalcms.com/tour/