Specifying type sizes for the screen is highly variable due to the differing sizes of monitors, native resolutions and physical pixel size. Our recommendation for type size on the screen is based on the pixel size of .25mm; a common size for pixels in current LCD displays.
For people with low-vision we recommend a minimum of 14 points or about 19 pixels set in Verdana is a comfortable measure. Traditional print sizes for text 9, 10 to 12 point, are too small for use on the screen. While this larger size will certainly not accommodate the most severe cases of low-vision, many people with this disability will benefit from this large size. Older adults will appreciate this larger size as well thereby improving their screen access (Bernard 2001).
X–height can be used as a method to specify type size. National Institute of the Blind (2006) recommends type sizes for low–vision have an x–height between 2mm and 4mm (2). This translates approximately as a typeface between 12 and 14 points (or 19 pixels), but be aware that the height of an ‘x’ in your chosen font is a more accurate measure. While these data are for print design we feel that they are a good starting point for screen typography.
The apparent type size is a factor of the x–height. In the same size, there is a factual and perceptible difference between Verdana, Arial and Times New Roman. Verdana has the highest x–height. This x-height effect on legibility becomes pronounced at the smaller sizes.
For navigational hyperlinks, small captions, credits, etc., when economy of size is important, it is acceptable to use smaller size, but not smaller than 75% of 19 pixel Verdana. Allow ample margin space around small type.At smaller sizes, well-spaced capitals can be more visible than lowercase letters, but they should be avoided for use of continuous reading. When selecting a font size keep in mind the scaling factor. Too small or too large a base value can result in legibility issues when scaling the computer screen. Scaling down can result in the smallest size becoming too small, and scaling up, the largest size can result in poor word breaks and an incomprehensible layout. (w3.org)