If you use the Custom format for Execute Sequence, you can change the values for any preset resolution button by right-clicking the button.
If you use one of the standard formats, the Width and Height spinners are locked to the standard's image aspect ratio, and the Aspect Ratio spinner is replaced by a text display.
After you change these values and exit the Configure Presets dialog, you must click the button to apply the new values to the Execute Sequence dialog.