Tracing the size of my file in my AIR app + tracing size of my server file?

I've got an xml file in my AIR app and I'm trying to determine his size. Here's what I did :

var file:File = File.applicationStorageDirectory.resolvePath("horaires3.xml"); 
trace(file.url); //path to the file trace(file.size); 

But I've got this error : Error #3003: File or directory does not exist.

The file.url is working, but the file.size is throwing the error..

2nd question (related) :

Can I check the size of a file in my server with AS3 code ?

Would it be something as simple as :

var myURL:URLRequest = new URLRequest("http://www.mywebsite/my_xml.xml");
trace(myURL.size);  

What I want to do is :

If (file.size == myURL.size)
{ //do nothing 
} 
else{ 
downloadmyURL(); 
}