Ok, após alguns man curl
de escavação, acho que descobri a resposta, embora não tenha 100% de certeza sobre sua validade:
-w, --write-out <format> Defines what to display on stdout after a completed and successful operation. [...] The variables present in the output format will be substituted by the value or text that curl thinks fit, as described below. All variables are specified as %{variable_name} and to output a normal % you just write them as %%. [...] size_request The total amount of bytes that were sent in the HTTP request. size_upload The total amount of bytes that were uploaded.
Isso significa adicionar um -w '%{size_request} %{size_upload}'
e, em seguida, adicionar os números resultantes na saída após o pedido fornecer o tamanho total da solicitação.