Wikipedia:WikiProject Usability/Readability guidelines

Size of text

Status: Under construction, but necessary informations are present. Needs to be organized into a guideline.

Reducing text size (for example, by 75%) causes usability and accessibility issues:

The first usability issues to consider is readability. An extensive number of usability studies were made about the impact of text size on readability. It was clearly stated that small text makes it way harder to read, for example by reducing reading speed: if a text with comfortable size would be read by average users in 15 seconds, the same small size text would be read in 27 seconds. The default font size on Wikipedia is already too small, so we don't want to make it even smaller.
The second and third issues are both usability and accessibility issues. First, users with low vision that have severe low vision (but are still able to read really large fonts) use dedicated software to enlarge or reduce the text size. So they will be able to read it. But when the normal text size changes to a very small text size, they may have to adjust their zoom level only for this particular text, and readjust it afterwards. It's suboptimal, but not detrimental.
The biggest usability issue is for elders (and possibly anyone older than 40), normal users with small low vision and normal users with a particular resolution, device, and such. Truth is, most of these users doesn't know how to zoom with their browser. Yes, they don't know "Ctrl +/-" even exist, or they forget it. So when they encounter such small text they have trouble to read, their only option is to come very close to the screen and strain their eyes as much as they can. Until they succeed or give up because of too much strain.

It would be way easier for everyone to not have to think about such issues, and simply don't ever reduce the font size. It also removes unnecessary code.

Resources on readability

Small text affects readability a lot, and has been thoroughly studied by usability experts: small text is way harder to read for everyone. Usability guidelines recommend a default font size of at least 12 points (about 16 pixels, but pixels are evil).

There is any number of useful resource about readability on the Web. But those three stand out:

An overview of readability best practices
Research on readability, for advanced users or Wikiproject members
List of further references