Font size in maths mode (math mode)

Use the following commands to change the font size in maths mode [1,2]:

{displaystyle }
size for equations in display mode
{textstyle }
size for equations in text mode; this will make the equation font size the same as the surrounding text
{scriptstyle }
size for first level of sub- and superscripts
{scriptscriptstyle }
size for subsequent sub- and superscripts

Example

The font size of the equation $latex f\left(x\right)=x$ can be changed to fit with the surrounding text, or it can have a superscript in a smaller font with a superscript in an even smaller font.

\documentclass[a4paper,12pt]{article}

\begin{document}

\huge

This function $\displaystyle f\left(x\right)=y$ is the same \texttt{\char\\huge} size as its surrounding text.

\small

This function $\textstyle f\left(x\right)=y$ is the same \texttt{\char\\small} size as its surrounding text.

\normalsize

This function $f_{\scriptstyle i}\left(x\right)=y$ is \texttt{\char\\normalsize} and contains \texttt{\char\\scriptsize} text ($i$).

\large

This function $f_{\scriptstyle i_{\scriptscriptstyle j}}\left(x\right)=y$ is \texttt{\char\\large}-sized and contains \texttt{\char\\scriptsize} text ($i$) and \texttt{\char\\scriptscriptsize} text ($j$).

\end{document}`

References

1. Roberts, Andrew, 2011, Getting to grips with LaTeX: mathematics – part 2