Let BorderContainer set the size of Dialog (and not the dialog itself): jsFiddle .
var layout = new BorderContainer({ design: "headline", style: "width: 400px; height: 400px;" });
In addition, in your case, you may not use dijit.layout at all. Place the contents of the dialog box inside the <div class="dijitDialogPaneContentArea"></div> and buttons in the <div class="dijitDialogPaneActionBar"></div> , and you will get:

Source: http://archive.dojotoolkit.org/nightly/dojotoolkit/dijit/tests/test_Dialog.html
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