I can't imagine that I have a narrow textarea with a narrow preview of what is supposed to be my post. Since the width wouldn't fit, it wouldn't even work as a preview. Maybe it would for plain text (but who needs preview for plain text), but as soon as you include images, headers, code snippets etc., the width starts to play a very important role.
My approach is that I learned how the markdown syntax works, including nuances like "a code block inside a list or a quote" and "a backtick inside an inline code", and "lists nested in lists", and since then, I don't use the preview but to check that code blocks and images fit in the page. Changing the width would remove this feature of the preview.
Considering the drawing that has appeared with the question: The 660/1440 px idea is plain wrong. The right panel that contains useful stuff (especially the Similar Questions, if only more people clicked couple of them before hitting "Post Your Question", the number of dupes would drop significantly), and you have to count it as well. The page width is 980 px, This means that a 1600-pixel-wide screen is actually not enough.