fwiw, these were my tunings back when i was using hydra and it was creating (larger) disk images:

  462       max_output_size = ${builtins.toString (1024 * 1024 * 1024 * 16)}
  463       #512 megs of logs
  464       max_log_size = ${builtins.toString (1024 * 1024 * 512)}
  465       #nar_buffer_size = ${builtins.toString (1024 * 1024 * 1024 * 6)}