you can get the size of the window or document using jQuery methods:
$(window).height(); // returns height of browser viewport
$(document).height(); // returns height of HTML document (same as pageHeight in screenshot)
$(window).width(); // returns width of browser viewport
$(document).width(); // returns width of HTML document (same as pageWidth in screenshot)
A non-jQuery way to get the available screen dimension. For responsive webdesign and completeness sake I think its worth to mention those attributes:
alert(window.screen.availWidth);
alert(window.screen.availHeight);