I have the same issue which has been discussed here while trying to scale plots created using PGFplots. The issue is only present if I name the external pictures using \tikzsetnextfilename. However, naming them would be very helpful, otherwise all external images would be recreated if I added or removed one of them (right?).
The OP used a custom made script to scale the tikz picture, while I use the tikzscale package. In the manual, it says the external library is supported, but after compilation, the picture is printed unscaled.
Can someone confirm, that this is supposed to be working "out of the box", or do I have to tweak anything?
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize
\usepackage{pgfplots}
\usepackage{filecontents}
\usepackage{tikzscale}
\begin{document}
\begin{filecontents}{bla.tikz}
\tikzsetnextfilename{pic}
\begin{tikzpicture}
\begin{axis}
\addplot coordinates {(0,0) (1,1)};
\end{axis}
\end{tikzpicture}
\end{filecontents}
\noindent\rule{\textwidth}{2pt}
\includegraphics[width=\textwidth, axisratio=2]{bla}
\end{document}
