Hit "cntrl+0" on your keyboard. This re-sizes your screen to it's original size. Does your background fit on wide screens? Not everyone's backgrounds fit wide monitors, but I'm happy to say that Dotty Dot Dot's backgrounds do!
Take a look:
Blog with a Dotty Dot Dot background - no white space on the sides.
Blog with a background from another site - white space on the sides on a wide monitor.