Guys in one of my recent interview I was asked about my project and in that he asked me what was the reason behind choosing the font size you have chosen for your app? I wasn't able to give a satisfying answer. Could someone help me how to know what should be the perfect font size?
Ergonomic criteria suggest that the appropriate font size is dependent on the luminance of the display screen and the importance of the information being presented (Sanders and McCormick, 1993). For critical uses (i.e., alarm messages) at 70 cm viewing distance, the font size on a display should be 5.1 to 7.6 mm for low luminance; and 3.0-5.1 mm for high luminance. Bullimore et al.(1992) recommends that alphanumeric character heights should subtend an angle of 20 min of arc, or 4 mm at a viewing distance of 60 cm. (source)
Not particularly a paper that answers the question directly, but this adequately sums up the main points. For screen sizes, these measurements can be converted to points and then to pixels. Mobile phones and tablets are held closer than laptops and desktops.
72 points = 1 inch = 2.54 cm
0.75 point = 1 px
The UX designer is concerned with legibility, how easily the text can be read and scanned. We, as UX professionals, are concerned about font-size, white space, font/background contrast and perhaps x-height. Ultimately all those issues are considered less for their aesthetic characteristics than for their ease of use for the end-user.
All my UX designs, unless there is an existing style guide, are done in Arial. Why? Because it's up to the graphic team, marketing and business to approve a style guide.
If you're applying to a UX role which encompasses graphic design work then the question "why did you use this font?" may have been appropriate. However if the position is strictly a UX role, and you didn't use an unusual combination on fonts, then a good UX answer would be:
"This is a common font that doesn't draw attention to itself. Under normal business circumstances I would follow the existing style guide."