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 my 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 indeed appears to be around 6mm tall on the screen.
A 6mm 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 every good eyesight.
For a screen with a diagonal that is X times larger, all these numbers multiply by X. For example, for a screen diagonal of 8 meters, 18pt (and also 16pt) should still be reasonably well visible from a distance of 20 meters.
RULE OF THUMB: If your smallest font size is 16pt, no member of the audience should sit 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.