There are some odd posts and misconceptions in this thread. It appears that many people want to ignore the science and the well researched aspects of how we read and comprehend text. As it happens, I'm in the process of researching new standards on this very subject, so here's a bit of the current state of the art:

The Science of Art

Existing research has defined that optimum reading speed for normally sighted individuals, with a font at maximum contrast, is an x-height between 0.2° and 2° of visual angle. X-height is the actual vertical size of the lowercase x of a font. Obviously the actual font size will vary based on the viewing distance, but fortunately the CSS reference pixel—px—is based on visual angle. A visual angle of 0.2° is known as the critical print size, as that's the point where maximum reading speed is achieved. (Above 2°, it goes back down.)

One px is 0.0213 degrees or 1.278 minutes of arc. This is based on a device with a pixel density of 96dpi @ distance 28 inches. Device manufacturers can thus use the reference px to set a size based on intended/expected visual distance. This is discussed on the W3C CSS standards. Device manufacturers use the reference px to set actual rasterizing size based on intended distance of viewing. 16px will not necessarily be 16 device pixels. On an iPhone with a 2:1 pixel ratio it would be 32 device pixels for instance.

Thus, the critical print size for the web is an x-height of 9.4px. Depending on the specific font design, this relates to a font between 17px to 20px. This resulted in accessibility standards that indicate that 18px is the minimum desired font size.

But Wait There's More

There is also a critical contrast level. The above mentioned font sizes relate to a maximum contrast. But what about for lower contrasts? Many designers are seriously impacting the readability of their sites by using lower contrast colors. Part of this is due to the failure of WCAG 2.0 in specifying correct contrasts relative to spatial frequency. 4.5:1 is more than needed for a big fat headline, but 4.5:1 is insufficient for small thin body text.

For normally sighted, critical contrast could be as low as 10% for big fat headlines at the peak of the contrast sensitivity function. But at the very high spatial frequencies of small thin fonts, contrast needs to be 20+ times higher. See the following diagram, where all text is in the same CSS color (and this is not even discussing the way antialiasing mangles text contrast beyond all recognition).

Thus, Font size along with contrast and a number of other design features work together provide a "most readable" text. Regardless, a 12px font size that many posters in this thread are recommending is shockingly too small, and where they came up with that figure is anyone's guess. You could use something like 12px for perhaps a copyright notice or something that you don't want anyone to read, but 12px is by no means an appropriate size for content text.

The Glyphs Have It

The official recommendation is the minimum size of 18px, nevertheless some fonts such as Verdana (a font designed for web use) may work well down to 16px. But Times New Roman should never be set at less than 18px as it has a very small x-height, and generally poor readability (thanks Microsoft, ugh). For another font that Microsoft mangled, let's try not to ever use "Courier New", where Microsoft took what was a generally readable monospaced font and then made it far too thin and light. Like, what were they thinking?

For some general considerations on font choice for accessibility and readability, I have this preliminary PDF on my research gate account that you can download for free: https://www.researchgate.net/publication/336679010_Evaluating_Fonts_Font_Family_Selection_for_Accessibility_Display_Readability

A Most Important Closing Comment

More important than what you set as a font size, is that you allow users to ZOOM text to whatever size they want without breaking content. The current standard specifies zoom 200% without breaking, but that is insufficient. 500% is much more reasonable from the user's perspective.

CONSIDER:

20/20 is average vision. The font sizes I mention above (18px) are based on this average 20/20 user. 20/40 needs TWICE that size for the same perception. 20/200 users need TEN TIMES that size (i.e. they may want to zoom 1000%). I mention 500% as a minimum as that considers the implications of the fact the page has larger fonts on it as well as the device physical size.

A technology that is missing that is in research right now is zooming up smallest fonts but zooming up the larger fonts less, so that large headlines don't become too big for readability.

In the meantime, just consider that a large portion of the people that are reading your site do not have a monitor as good as yours, and do not have eyesight as good as yours. If you want and idea of how your site might be seen by some less fortunate, get a cheap, junky, small, low res monitor (you might have one in storage), and set it 3 to 4 feet away (i.e. more than a meter away). Can you read your site? Now zoom the text—does your site break because of the confines of the small monitor?

This is the kind of issue many users have with many sites. Just because you have 20/15 vision and a beautiful 32" retina display does not mean that your users have nearly that level of visual accommodation.

-Andy