Best font size for Latex Beamer

The font should be generously large, so that everyone can see it: for typical talk situations font size is not the main constraint, but amount of information being displayed per size.

The advice I generally heard has been to have very few slides with more than 10 to 12 lines of content on them, including the slides heading. The beamer manual suggests that its default of an 11pt font (onto a quite small page, so this is not as small as it sounds) leads to a maximum of around 20 lines on the page, which seems to be rather too high.

By contrast Powerpoint uses a default font size of 17pt, which gives much better amount of information, but often seems to lead, to my taste, to somewhat bombastic presentations.

The middle of this range, around 14pt, seems a good median.

Some further considerations:

1. I am not a beamer expert. I usually handwrite my slides!
2. You can justify using rather bigger fonts, to give a point some punch, and sometimes you have a complex idea that is best presented all at once. Variations in the amount of information of the slide should be accompanied by variations in how you present that slide: this is easy with less information, but the risk with more information is that you don't guide the audience through all of what you are presenting properly.
3. You generally should read out everything on the slide. If this seems silly for some piece of content, it probably shouldn't be there. Don't be tempted to fit large pieces of code on a slide and expect the audience to just absorb what is there: chop out the bits that matter and explain them properly.

When making a presentation your concern should be the readability of projected text. Terms such as 12pt, 16pt are really for type on paper. When projected your text might be 10cm in x-height on the screen in a large hall. There is no good alternative to rehearsing in the room you will be making the presentation in, and checking the legibility of text from the furthest seats.

There are a lot of factors which make rules about type on paper inadequate for guidance on type on a screen. Some of these are:

• the difference between the high resolution of laser printers and the generally lower resolution of projectors;
• foreground and background colour combinations which work well on paper may work very poorly on screen, and vice-versa;
• even the quality of the projection screen has an impact on the readability of text.

This will vary greatly on your screen resolution and on the size of the projection screen.

Charles and I clearly disagree! I've just beamer a fair few times for talks and just left the default sizings as they were and got good results. I believe Beamer is designed with a similar mindset to LaTeX: it uses defaults that are good enough for the intended use.

In my talks, I use Futura as my main font, which is very readable at large sizes. Relevant bits of my standard preamble follow. (I use xelatex btw, which makes font switching much easier)

\usetheme{Copenhagen}
\usefonttheme[onlymath]{serif}
\usepackage{fontspec} 
\defaultfontfeatures{Mapping=tex-text} 
\setsansfont[Ligatures={Common}]{Futura}
\setmonofont[Scale=0.8]{Monaco}