如何从Golang中的URL检索HTML文档大小

I used GoQuery to retrieve some value within an HTML document. Now I need to get the size of HTML document (without assets).
In Firefox, it's as simple as Tools --> Page Info (General) tab show the Size of the HTML document.
I tried net/html package as well but I can't find the size of the returned HTML given a URL. Any clues?

Get the document via:

res, err := http.Get(url)
if err != nil {
    // handle error
}
defer res.Body.Close()

Now you can:

body, err := ioutil.ReadAll(res.Body)
if err != nil {
    // handle error
}
l := len(body)

To use with GoQuery:

utfBody, err := iconv.NewReader(res.Body, charset, "utf-8")
if err != nil {
    // handler error
}

doc, err := goquery.NewDocumentFromReader(utfBody)
if err != nil {
    // handler error
}

More datail about GoQuery and readers is here.