With tcolorbox version 3.90 (2016/02/29), there is a property \tcbtextheight which holds the text height of a fixed height box (inspired by this question!). With it, the solution is short:
\documentclass{article} \usepackage{pgfplots} \usepackage{tikzscale} \usepackage{tcolorbox} \tcbuselibrary{raster} \begin{filecontents}{tikzimage.tikz} \begin{tikzpicture} \begin{axis}[xlabel=time,ylabel=value] \addplot{x^2}; \end{axis} \end{tikzpicture} \end{filecontents} \begin{document} \begin{tcbraster}[% raster columns=2, raster rows=2, raster height=\textheight, ] \begin{tcolorbox}\end{tcolorbox} \begin{tcolorbox}\end{tcolorbox} \begin{tcolorbox}\end{tcolorbox} \begin{tcolorbox}[title={box 1\\line two}] \includegraphics[% width=\linewidth, height=\tcbtextheight, ]{tikzimage.tikz} \end{tcolorbox} \end{tcbraster} \end{document} Old Answer (valid for tcolorbox before version 3.90):
In my answer, a new option remember height is constructed which saves the actual inner height into a chosen macro at the begin of the upper box. The following example uses \myheight which can be used to scale the included image.
