rargh shapes & other design elements!

So, if I wanted a dreamwidth journal layout that kept entries in a fairly narrow column, but that filled a whole screen - so that changing the width of my browser window would have no effect on the width of displayed entries - what would that be called and where would I find it? I tend to browse the net with my browser window either fullscreen or narrowed, bouncing back and forth - blogs go fullscreen, fic posts and such narrower, because I like fairly short lines of text if I'm reading a lot of it, and at present my dwidth layout is built for a narrow view & looks cruddy expanded.

eta: found something, have been poking css. In the process, I seem to have lost the css snippet I was using to start my header image at the far left of the page and extend it across the entire top of the screen. Argh. any ideas?