You could scale / using \scalebox{<factor>}{<stuff>} from the graphicx package:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\begin{document}
\[\left.
\begin{vmatrix}
a_{11}&a_{12}&a_{13}\\
a_{21}&a_{22}&a_{23}\\
a_{31}&a_{32}&a_{33}
\end{vmatrix}\middle/\begin{vmatrix}
b_{11}&b_{12}&b_{13}\\
b_{21}&b_{22}&b_{23}\\
b_{31}&b_{32}&b_{33}\\
b_{41}&b_{42}&b_{43}
\end{vmatrix}
\right.\]
\[
\begin{vmatrix}
a_{11}&a_{12}&a_{13}\\
a_{21}&a_{22}&a_{23}\\
a_{31}&a_{32}&a_{33}
\end{vmatrix}\scalebox{1.5}{\Bigg/}\begin{vmatrix}
b_{11}&b_{12}&b_{13}\\
b_{21}&b_{22}&b_{23}\\
b_{31}&b_{32}&b_{33}\\
b_{41}&b_{42}&b_{43}
\end{vmatrix}
\]
\end{document}
Note that this scales / both vertically and horizontally, increasing the width of the symbol as well. To minimize this, I've only scaled \Bigg/. To see the difference, consider (say) \raisebox{-2ex}{\scalebox{4.5}{/}}.
Another alternative could be to rotate a vertical rule by (say) 15 degrees clockwise. For this you could use
\rotatebox{-15}{\rule[-1.8\baselineskip]{0.4pt}{4\baselineskip}}
The above vertical rule has a fixed width of 0.4pt, which you can change to your liking.