Equal font sizes in equation while using Beamer

The default text size used by beamer is 11pt, which must be referred to as \@xipt. Just for an example I tried very big math font sizes, using the Arev fonts to overcome problems in font loading:

\documentclass{beamer}
\usepackage[T1]{fontenc}
\usepackage{arevtext,arevmath}
\usefonttheme{professionalfonts}

\makeatletter
\DeclareMathSizes{\@xipt}{50}{40}{30}
\makeatother

\begin{document}
\begin{frame}
Text $m_{a}+t+\sqrt{h}$
\end{frame}
\end{document}

Similar declarations should be made for the other sizes, if you plan to use math in \large, \small or similar contexts.

However, I don't think that changing the math sizes is a good thing to do.

---

After seeing the minimal example and the real problem you have, I suggest you to use the \cfrac command provided by amsmath.

\documentclass{beamer}
\usetheme{Warsaw}
\usepackage{amsmath}
\begin{document}

\begin{frame}
\frametitle{test equation to have a same font size}
    \[
           x = \cfrac{1}{x + \cfrac{1}{x + \cfrac{1}{x}}}
    \]
\end{frame}

\end{document}