[Tex/LaTex] Using powers in \dimexpr

lengths

The code

\documentclass{memoir}

\begin{document}
\fontsize{\dimexpr 1.2^{3} 10pt}{\dimexpr 1.2^{3} 10pt}\selectfont Hello TeXworld.
\end{document}

does not work. Why not?

EDIT: I need a solution that supports all integral (including negative) exponents.