the problem seems to be that you have a lot of code in that area that doesn't belong where it is.
you have a php script called in a way that probably doesn't work, you have whatever <ins is wrapping the whole thing.
you have the navbar inside the headerbar div and you have another php script there that is not doing anything because you have set it to be hidden.
so, I have no idea how you would change the font size for that text alone.
and yes, if you put a <h4> tag somewhere then you have to use the closing </h4> tag when you want it to stop working.
also, the css for the h4 tag it is listed in common.css at the top so you will need to edit it for whatever you want it to do.
I am available for custom work on a donation basis. Please send me a PM with your needs.
Premium phpBB 3.2 Styles
OK, so what's the speed of dark?