First, please start your own thread instead of extending an existing one. If you need to refer to another thread, you can always add a link that points to it.
Second, one thing you should not do is go to Appearance → Editor and edit any theme files. If the theme gets updated because of feature enhancements, bug fixes, or security patches, or if the theme has to be updated because of a change to the WordPress core, then your changes will be lost. Instead, either create a child theme or use a CSS plugin like Jetpack or Custom CSS Manager. Since all you need is to add some CSS, then it would probably be easier for you to use a plugin.
If you provide a link to your site, then I can give you more specific suggestions on how to make your logo smaller.