Hi Sumeet,

Refer the below link. You need to set the height of the modal dialog in the script like below also if you want to make resizable then make the property as true.

Validation Summary in jQuery Dialog Modal Popup Box not working with ASP.Net AJAX UpdatePanel

Javascript

<script type="text/javascript">
    function OnSubmit() {
        if (typeof (OnSubmit) == "function" && Page_ClientValidate() == false) {
            $("#validation_dialog").dialog({
                title: "Validation Error!",
                modal: true,
                resizable: false,
                height: 200,
                buttons: {
                    Close: function () {
                        $(this).dialog('close');
                    }
                }
            });
            return false;
        }
        return true;
    }
</script>

Screenshot