[R] Plot symbols dimensions

[Cressoni, Massimo (NIH/NHLBI) [F]]

I am writing some code to obtain publication-like plots (like the ones can be obtained with
SigmaPlot). I am not able to find the dimensions of a point. Parameters like cex make you able 
to make it bigger or smaller but I need to know the exact dimension (in relative coordinates :
if my plot is 640x480 a dot plotted with cex=1.5 will be...)

Massimo Cressoni