There are several ways to do this:
- You can use
\scalebox from the graphicx package.
- Use the
relsize package.
- Use
\DeclareMathSizes but that will affect the entire document.
Here is the normal, and scaled result using the \scalebox solution:

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}
\newcommand*{\Scale}[2][4]{\scalebox{#1}{\ensuremath{#2}}}%
\begin{document}
\[y = \sin^2 x\]
%
\[\Scale[6]{y = \sin^2 x}\]
\end{document}
Or, using the relsize package, but it did not appear to get larger using a factor larger than 5. Perhaps there is a way to adjust this.

\documentclass{article}
\usepackage{amsmath}
\usepackage{relsize}
\begin{document}
\[y = \sin^2 x\]
%
\larger[5]
\[y = \sin^2 x\]
\end{document}