Hey Anon,
I tried this and its working perfect
var qwe = xtk.queryDef.create(
<queryDef schema="nms:delivery" operation="select">
<select>
<node expr="@id"/>
<node expr="[content/sms/source]" alias="@content"/>
</select>
<where>
<condition expr="@id=157651363"/>
</where>
</queryDef>
);
var result= qwe.ExecuteQuery();
for each(var res in result){
var tempValue=res.@content.toXMLString(true).length;
logInfo('str'+tempValue);