I'm using to increase the single letter the package relsize at the page 1:
\documentclass{article}\usepackage{relsize}\begin{document}${}^{A}_{Z} {\mbox{\Large$X$}}_N$, \quad ${}^{A}_{Z} {\mbox{\larger[3]$X$}}_N$, \quad ${}^{A}_{Z}{\mbox{\larger[6]$X$}}_N$\end{document}The output is:
