You can use $(window).height() * 0.9
to get 90% of a window, and also set max height dynamically when dialog opens (in case window was resized/rotated).
And.. looks like it does not respect maxHeight property when dialog does not have fixed height (height:auto), so you can set it via max-height css property, after you resize dialog first time it should obtain fixed height and maxHeight will work from that time
$("#div_dialog").dialog({
title: "Header",
maxHeight:$(window).height() * 0.9,
open:function(event, ui){
$(this).css("max-height", $(window).height() * 0.9);
}
});