Beamer: how to change the font size in math mode?

Question

The first image is what I have at the moment, the second image is what I would like to get. As you can see, in the second image the formula font size is a little bit bigger than in the first image. I would like to make all the formulas font size a little bit bigger. I tried using, as suggested here, \DeclareMathSizes{14}{15}{7}{7} (note that in beamer I am using 14pt as font size) but it did not produce any effect.

\documentclass[14pt,handout,t]{beamer}
\usepackage{lmodern}
\usepackage[english,italian]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\geometry{paperwidth=297mm,paperheight=210mm}
\usefonttheme{professionalfonts} 
\renewcommand{\sfdefault}{phv} %change font
\DeclareMathSizes{14}{15}{7}{7}

\begin{document}
\begin{frame}
complesso: $\alpha=\alpha_{r}+i\alpha_{i}$
\end{frame}

\end{document}

Answer 1

In your linked question Yiannis writes:

When using the \DeclareMathSizes the first parameter for the text font must be exactly what is used in the class declaration. For example with Beamer rather declare \documentclass[10pt]{beamer}, otherwise the command will appear as it is not working.

That is not a hundred percent correct. You have to use the font size you actually use at the moment. You can output it in the log with

\makeatletter
\show\f@size
\makeatother

and in your MWE this shows

> \f@size=macro:
->14.4.

So the beamer option 14pt uses actually a 14.4pt font size and you have to use

\DeclareMathSizes{14.4}{15}{7}{7}

or

\makeatletter
\DeclareMathSizes{\f@size}{15}{7}{7}
\makeatother

that it works.