How to decrease the size of font of categories???
It depends on the theme but from a previous post (http://forum.coppermine-gallery.net/index.php/topic,54701.msg266535.html#msg266535) you made, it looks like you're using the Dfire theme I ported so this is controlled by the h2 style. In future post please include a link to your gallery so we can offer better help and check out this sticky on web development tools (http://forum.coppermine-gallery.net/index.php/topic,31423.0.html). These tools make this type of questions unnecessary.