\documentclass[12pt]{article}
\thispagestyle{empty}
\usepackage{amsmath}
\begin{document}
\huge{
$\psi( \vec{r},t) = \psi_0 \exp\left(i\left( \vec{k}\cdot\vec{r} - \omega t \right) \right)$
}
\end{document}
