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.