Below is code through which you'll be able to get size of the downloadable file
string completeUrl =
"TypeYourDownloadbleURL";//Please write the exact URL of file
WebClient obj = new WebClient(); //New object of WebClient
Stream s = obj.OpenRead(completeUrl);
Response.Write(obj.ResponseHeaders["Content-Length"].ToString());//Get the size
obj = null;
I hope that helps you.