Please consider this minimal example. Compiled with XeLaTeX.
\documentclass[fontsize=14pt]{scrartcl}
\usepackage{unicode-math}
\setmathfont{XITS Math}
\begin{document}
Test $\exists a'$.%^\prime: same.
\end{document}
As you can see the "prime" is too high (above the \exists symbol). The problem does not happen when using fontsize 12 pt or removing \setmathfont.
Any idea what’s wrong? What is the recommended way of changing the default font to 14 pt? I thought about using Scale (as suggested here) but I’m not sure scaling a big amount (one sixth) is recommended.