Page size and browser window size are not the same thing. A browser window can be as larger as an entire monitor’s display area (and on a 30" monitor that is HUGE) but there is no way of knowing this ahead of time. You can preset a popup window size, but the user can always change that once it’s opened.
Think of a “page” area as the minimum area you want the primary content to display in. The rest is just background area.