I have used it for years, actually, bak to my days of playing live. I never liked my windows being in the way of seeing mobs or looting. Pretty hard to screw it up, though. It is the viewport command, followed by x offset, y offset, x size, and then y size. For example, /viewport 50 0 1200 800 will give you a 1200 by 800 window for view, offset from the left by 50, and starting right at the top. I found it best to work in small increments off a base like that, until you found the right sizing for you.