CURLOPT_BUFFERSIZE man page
CURLOPT_BUFFERSIZE — set preferred receive buffer size
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_BUFFERSIZE, long size);
Pass a long specifying your preferred size (in bytes) for the receive buffer in libcurl. The main point of this would be that the write callback gets called more often and with smaller chunks. Secondly, for some protocols, there's a benefit of having a larger buffer for performance.
This is just treated as a request, not an order. You cannot be guaranteed to actually get the given size.
This buffer size is by default CURL_MAX_WRITE_SIZE (16kB). The maximum buffer size allowed to set is CURL_MAX_READ_SIZE (512kB).
Added in 7.10. Growing the buffer was added in 7.53.0.
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.