Thanks for this.
Just want to add (or clarify) that you can also use different unit for the max-width, too.
For example, when I zoom the font really big, I want to reduce the length to 35rem (default is 40 I think).
.markdown-preview-view.is-readable-line-width .markdown-preview-sizer, .markdown-source-view.is-readable-line-width .CodeMirror
{ max-width:35rem; }
Still, for it to become a stand-alone setting and be able to quickly adjust would be nice.