Use the Fractal Architect menu -> Preferences to open the Preferences dialog. Select the Preview pane and click the button between the width and height field. 

If the aspect ratio is constrained, the button will show “--”. Then changing height or width will automatically change the other setting to maintain the current aspect ratio. It also forces preview window resizes to maintain the aspect ratio.

If the aspect ratio is unconstrained, the button will show “<    >”. This allows you to change width and height independently. Bug alert: the preview window still preserves the aspect ratio.

Note that once the file is opened in the preview window, you can unlock the aspect ratio, resize the window to a new aspect ratio, then re-lock it. This does not change the Preference setting however.