What is Auto Zoom?
Auto Zoom (sometimes written as AutoZoom) is a function in MultiMarkdown Composer that automatically adjusts the zoom level so that your document uses all available space.
You specify your desired line width (in characters) in the preferences (80 in this image):
Using the above example, Composer would adjust the zoom level such that 80 characters in whatever font would fit properly in the window, regardless of how large the window is. This is useful when using Full Screen mode, for example, so that your text makes best use of the extra screen space.
Note: Style themes can enable (or disable) Auto Zoom, and can specify a line width. If so, these settings may override what you set in the preferences. In this case, you may wish to modify the theme to suit your own needs. (More information on customizing style sheets is here).
You can disable Auto Zoom with the "View:Auto Zoom" menu command, or keyboard shortcut. It is also automatically disabled if you zoom in or out.