>Actually, no more so. You have gradually removed bits and pieces of content from the From and To boxes, and these can be made narrower by 40 pixels each without much damage. If some pieces start wrapping, so what - the owners of narrow screens know they'll suffer some reshuffling of content because of their screen of choice. The middle section is even easier, leaves few hundred pixels horizontally. Same for the toolbar on the bottom, there's enough whitespace to squeeze that by a couple of hundred pixels if need be.
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>I'd suggest you allow 800px wide as an experiment, and I volunteer Greg and me to test it.

Each message is dynamic. Some have a lot of content on the upper right, just above the message but under the To rectangle. All that, when all filled, leaves no more room for anything else.