specifying font size in R figures
You control the font size with the ps (point size) parameter. The default is typically 12 (but can be controlled globally for a PDF file by the pointsize parameter) so if you want, let's say, fonts of the size 10 for a particular text you would use par(ps=10); text(...). Since you mentioned cex: note that cex is relative to the current pointsize and also applies to symbols whereas ps specifically applies to text. Obviously, the size will only match as long as you don't resize the resulting figure.