You can try to resize the dialog using its classes by JQuery directly (documentation here)
The basic structure of JQueryUI Dialog is this:
<div class="ui-dialog ui-widget ui-widget-content ui-corner-all ui-draggable ui-resizable">
<div class="ui-dialog-titlebar ui-widget-header ui-corner-all ui-helper-clearfix">
<span id="ui-dialog-title-dialog" class="ui-dialog-title">Dialog title</span>
<a class="ui-dialog-titlebar-close ui-corner-all" href="#"><span class="ui-icon ui-icon-closethick">close</span></a>
</div>
<div style="height: 200px; min-height: 109px; width: auto;" class="ui-dialog-content ui-widget-content" id="dialog">
<p>Dialog content goes here.</p>
</div>
</div>
So, maybe you should play with classes's width and height to set the best.
Another solution is to set dialog's width directly before open (when your data is successfully loaded):
$("#a_div").dialog({ width: 400 });
$.get('my_url.php',function(data){
$('#a_div .ui-dialog').css('width','400px');
// Or...
$('#a_div').css('width','400px');
});
I hope it helps you.