Mega Code Archive
Use width and height properties to explicitly set its size
<![CDATA[
<?xml version="1.0"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:HBox id="myHBox">
<mx:TextInput id="myInput" width="200" height="40" />
</mx:HBox>
</mx:Application>
]]>