beamerfontsize
I can adjust the default font size for a Beamer presentation using any of these options:
\documentclass[pdf,12pt]{beamer} % very small
\documentclass[pdf,14pt]{beamer} % small
\documentclass[pdf,20pt]{beamer} % very large
Values between 14pt and 20pt give the same result as 12pt, but I think the optimal font size lies somewhere between. How can I adjust the font size to something like 18pt?
itemize symbols have font size hardcoded in definition (look at beamerthemedefault.sty or beamerbaseauxtemplates.sty).
% Itemize items, circle
\defbeamertemplate{itemize item}{circle}{\small\raise0.5pt\hbox{\textbullet}}
\defbeamertemplate{itemize subitem}{circle}{\footnotesize\raise0.5pt\hbox{\textbullet}}
\defbeamertemplate{itemize subsubitem}{circle}{\footnotesize\raise0.5pt\hbox{\textbullet}}
Section 16.3 "Changing the Templates Used for Different Elements of a Presentation" form beamer manual explain how to change it. It seems that would be possible to define a template with an option for font size. I don't know how to do it but a fast solution could be redeclare itemize symbols with new hardcoded size, something like
\documentclass{beamer}
\begin{document}
\begin{frame}[t]{Frame title}
\begin{itemize}
\item First item
\item Second item
\end{itemize}
\bigskip
{\setbeamertemplate{itemize item}{\small\raise1.25pt\hbox{\donotcoloroutermaths$\blaktriangleright$}}
\begin{itemize}
\item Second first item
\item Second second item
\end{itemize}
}
\bigskip
\begin{itemize}
\item Third first item
\item Third second item
\end{itemize}
\end{frame}
\end{document}
EDIT: Changing balls for Madrid Theme.
Madrid Theme (beamerthemeMadrid.sty) uses \useinnertheme[shadow]{rounded} which declares \setbeamertemplate{items}[ball]. So we need to look for ball in beamerbaseauxtemplates.sty. To do it short, in this file some 'spheres' are defined and used. You can make your own definition,
\documentclass{beamer}
\usetheme{Madrid}
\makeatletter
\pgfdeclareradialshading[bg,parent.bg]{mysphere}{\pgfpoint{0.15cm}{0.15cm}}%
{color(0cm)=(bg!15);
color(0.15cm)=(bg!75);
color(0.3cm)=(bg!70!black);
color(0.301cm)=(parent.bg)}
\defbeamertemplate{itemize item}{myball}%
{\raise-0.2cm\beamer@usesphere{item projected}{mysphere}}
\makeatother
\begin{document}
\begin{frame}[t]{Frame title}
\begin{itemize}
\item First item
\begin{itemize}
\item first subitem
\item second subitem
\end{itemize}
\item Second item
\end{itemize}
\bigskip
{\setbeamertemplate{itemize item}[myball]
\begin{itemize}
\item Third first item
\item Third second item
\end{itemize}
}
\end{frame}
\end{document}
The result is
I think sphere sizes are related with font size because they are declared using ex units but I'm not able to change its size with just a fontsize declaration. May be somebody else can help us.
According to section 18.2.1 of the beamer manual, a default font size of 8pt is already "way too small". beamer uses the extsizes packages for non-standard font sizes, and extsizes doesn't offer anything smaller than 8pt. If you really must fit more text on each frame, I suggest to enlarge beamer's paper size from 128mm*96mm to, say, 140mm*105mm.
\documentclass[8pt]{beamer}
\geometry{paperwidth=140mm,paperheight=105mm}
\usepackage{lipsum}
\begin{document}
\begin{frame}
\lipsum[1]
\end{frame}
\end{document}
EDIT: Another option is to load the scrextend package (part of KOMA-Script) which allows to choose arbitrary font sizes like 7.5pt -- be sure to use a vector font like Latin Modern.
\documentclass{beamer}
\usepackage{lmodern}
\usepackage{scrextend}
\changefontsizes{7.5pt}
\usepackage{lipsum}
\begin{document}
\begin{frame}
\lipsum[1]
\end{frame}
\end{document}
Best Answer
There is no easy way: The list of font size options in beamer is hard coded. If you want other font sizes you should copy the definitions from e.g.
size17.cloto your preamble, change\newcommandto\renewcommandand then adjust the values.