The default configuration options you can find in FCKeditor documentation.
You can override almost of this options. Just create or edit your userconfig.js file (/editor/fckeditor/editor/userconfig.js). You can also edit this file via Control Panel - Editor Configuration.
FCKConfig.DekiTabSpaces = 4 ;
This option overrides the FCKeditor's configuration option TabSpaces and has the same functionality.
You can't change the following options: