If you wish, you can select or deselect any or all of the options which appear below the window size buttons. These options determine whether various optional screen features will be displayed in the new browser window. It is, however, recommended that they are all left selected because the majority of users will have them all selected.
If your screen is smaller than the window size, you can move the window by dragging the title bar at the top of the window. This way, you can see how your pages align even at resolutions bigger than your own screen can show at one time.