Font size in maths mode (math mode)

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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}