CURLINFO_SIZE_DOWNLOAD man page
CURLINFO_SIZE_DOWNLOAD — get the number of downloaded bytes
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_SIZE_DOWNLOAD, double *dlp);
Pass a pointer to a double to receive the total amount of bytes that were downloaded. The amount is only for the latest transfer and will be reset again for each new transfer. This counts actual payload data, what's also commonly called body. All meta and header data are excluded and will not be counted in this number.
Added in 7.4.1
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
curl_easy_getinfo(3), CURLINFO_HEADER_SIZE(3), CURLINFO_REQUEST_SIZE(3).