main-0.go

// We need a URL to be downloaded.
if len(os.Args) < 2 {
	fmt.Println("One more commandline argument needed. (url)")
	return
}

// assign it to a variable.
url := os.Args[1]

// I'm not checking the URL passed for validity or url query parameters.

// Make a head request for getting the file size.
head, err := http.Head(url)
if err != nil {
	fmt.Println(err)
	return
}
fmt.Println(head.ContentLength)