\documentclass{article}
\usepackage{sizesub}
\usepackage[30]{sizedef}
\usepackage{exscale}
\renewcommand{\familydefault}{\sfdefault}
\begin{document}
This is an example in normalsize at 30pt.
$$\sum_{n=1}^{\infty}\frac{1}{n^2}=\frac{\pi^2}{6}$$
{\SMALLsize 1} 2 {\LARGEsize 3} \textrm{A \textit{B} \textbf{C} \textit{\textbf{D}}}
\end{document}
