jQuery.ui.domwindow - demo - flexiblesize

width: $window.width() - 20
height: $window.height() - 20

But these do not get over width: 750 / height: 450.

Resize your browser then try.

ajaxdialog

open

$(function(){

  var $window = $(window);

  $.ui.domwindowdialog.setup({
    fixedMinY: 10,
    spinnersrc: '../../spinner.gif' // specify this for preload
  });

  var api = window.domwindowApi;

  $('#open').on('click', function(){

    var width = $window.width() - 20;
    var height = $window.height() - 20;

    // handle max,min width/height
    if(width > 750) { width = 750; }
    if(height > 450) { height = 450; }

    api.open('dialog1.html', {
      width: width,
      height: height,
      afteropen: function(e, data){
        console.log('afteropen!', data.dialog);
      },
    });

  });
});