>>If you've removed the right pane, then play with the message view setup (top right of this frame), set message area size. I've set mine to 880px, which is just about the width I like (and can still see when I move the browser to my right monitor, which is vertical, so it has only 1080 horizontally).
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>Attached is the bottom half of what I see when the width is set to 700 - it won't let me go any smaller than that. Keeps telling me that the value needs to be between 600 - 1400, even when I try to set it to 650

With people almost never trimming their quotes, I actually wanted to make it wider than the default, so I'd have less to scroll :).

And it's been years since I set it this way, so I don't remember what it took for my settings to take hold. Logging out, closing the browser, clearing the browser cache - no clue. I only remember I had to fiddle with it for a while.