This might be helpful

func CalculateRequestSize(req *http.Request) (int, error) {
    b, err := httputil.DumpRequestOut(req, true)
    return len(b), err
}

func CalculateResponseSize(resp *http.Response) (int, error) {
    b, err := httputil.DumpResponse(resp, true)
    return len(b), err
}