First written by Hannah Bast on 15.10.2019.
A common question I have when giving a presentation at another place is how large the screen size is, how far the audience is sitting away from the screen and whether they will be able to see everything on my slides. It is suprising how little attention is given to this question and how often it happens that much of the slides cannot be read by much of the audience because it's simply too small to read.
Here are some basic computations to alleviate this problem and to be able to judge which miminum font size is necessary for given screen dimensions.
On a screen with a resolution of 1920x1080 (16:9) and a diagonal of 77.71cm (30.6in), 1pt appears as exactly the size of 1pt on the screen, where 1pt = 0.352778mm = 1/72th of an inch. Here is a calculator for that.
That is, on such a screen, an 18pt character is 6.35mm large on the screen (16pt -> 5.6mm, 20pt -> 7.1mm, 24pt -> 8.5mm). I checked this on my notebook screen, which has half that diagonal, and with a magnification of 200% a 16pt character (I took a zero) indeed measures around 6mm.
A 6mm tall character can be seen very well from a distance of 1 meter and still well from a distance of 2 meters. For 3 meters you need to have good eyesight (like Hannah has not), and for 4 meters you need to have very good eyesight.
For a screen with a diagonal that is X times larger, all these numbers multiply by X. For example, for a 1920x1080 screen with a diagonal of 8 meters, 18pt (and also 16pt) should still be reasonably well visible from a distance of 20 meters.
RULE OF THUMB: With a 1920x1080 resolution and a minimum font size of 16pt, it is OK if no member of the audience sits further away from the screen than 2.5 times the size of the screen diagonal.
Corollary: If you can do nothing about the screen size or the distance of the audience from the screen, increase the minimum font size accordingly. It all works linearly: for example, a font size of 32pt is still reasonably well readable at a distance of 5 times the size of the screen diagonal.