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Getting file size in Silverlight 4
WebClient wc = new WebClient();
wc.OpenReadCompleted += Load_TransferCompleted;
wc.OpenReadAsync(uriAddress, Filename);

WebRequest req = WebRequestCreator.ClientHttp.Create(yourUri);
 req.Method = "HEAD";
 req.BeginGetResponse(ar =>
 {
     WebResponse resp = req.EndGetResponse(ar);
     int length = resp.ContentLength;

     // Do stuff with length
 }, null);