Assuming you're on GNU/Linux: (setq read-process-output-max (with-temp-buffer (insert-file-contents "/proc/sys/fs/pipe-max-size")

Assuming you're on GNU/Linux: (setq read-process-output-max (with-temp-buffer (insert-file-contents "/proc/sys/fs/pipe-max-size") (string-to-number (buffer-string))))