In the box “Position in the browser” you can set up the map’s size in the browser. Besides the option "Full screen" you can also limit the size pixel-accurately with "User defined". In order to do this, the corresponding values are to be typed into the adjoining entry boxes.
You can either specify the “absolute” values (i.e. in pixels) or select “relative” where the map size is set as a percentage of the browser window size. After that, the numeric values have to be entered into the entry fields grouped under “Position” and “Size”.