Você precisa recompilar o Nginx se quiser linhas de log com mais de 2048 bytes. A partir do link :
There is a hard-coded length limitation on the error messages in the Nginx core. It is 2048 bytes at most, including the trailing newlines and the leading timestamps. You can manually modify this limit by modifying the NGX_MAX_ERROR_STR macro definition in the src/core/ngx_log.h file in the Nginx source tree. If the message size exceeds this limit, the Nginx core will truncate the message text automatically.