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