1 - a link in includes/templates/YOUR_TEMPLATE/sideboxes/tpl_categories.php would work site wide
2 - Using FireFox you can inspect the code and click on the text you want to change. You will find the CSS that's contributing to the display size if it's not the default. Like sideBoxContent for example. If there is no font/size set - set one and work your way in. You may need to create a new one to handle the various sub/parent spans it's currently using.