About CSS and style

I'm new to this PunBB, I just started a forum, and can't figure out how to do what i want...

I like the Zoey style, but i have 2 "problems". 1- Font size is too small- both normal, and in quotes, 2- text is too narrow (if you spell it like that), and the space for info about user is too wide.
How can i change those things? I saw you posting here some codes for editing forum, smth about CSS.. But i don't know what that is... How do i edit it?

about the font size- i got used to phpBB- you just set when writing a post, [size][/size] and that's it..