This option sets the width of the built-in resource browser used in the "Image Properties" window. By default the width is expressed in percentage of the screen. E.g. 0.7 equals 70% of the screen.
FCKConfig.ImageBrowserWindowWidth = FCKConfig.ScreenWidth * 0.7 ;
Of course you may also express the width in pixels. Example:
FCKConfig.ImageBrowserWindowWidth = '220' ;