Usually 16px to 18px is best for blocks of large blocks of texts, anything bellow that becomes hard to read and can strain the eye, as 16px on screen is closest to the 12pt fonts used in printed media. Usually people tend to read larger fonts faster. It also depends on the type of font used. Some fonts look absolutely tiny at 16px and need to be set at around 18px or more.
You also have to consider that as people age, reading text set at 16px or 12pt gets increasingly difficult, thus, larger text is needed. Too large becomes a problem when users have to scan too many lines to get the full picture of text, too small and the user gets put off because the text looks too dense.
This article states much of this: http://www.smashingmagazine.com/2011/10/07/16-pixels-body-copy-anything-less-costly-mistake/
They use a size of 18px, but that's because the font they use looks tiny in smaller sizes.
This research paper states that larger font size tends to be faster to read, but that the font type has a big role to play as well.http://www.bcs.org/upload/pdf/ewic_hc08_v2_paper4.pdf