Text does not fill the page

I want the text to fill my whole page, but it does not. Can I adjust that?
See picture at http://f.cl.ly/items/3Y0C1q3U3P101X1l3o2m/Bild%201.png

It looks like you have fixed width turned on in your editor. Check the Editor preferences tab, middle section on the right-hand side. You should see an option for keeping the editor at a fixed width. Just disable that.