[R] Plot symbols dimension

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

I am looking a way to find the dimension of plot symbol in a graph in relative coordinates. 
Importing the graph in GIMP and measuring it can work but I am looking for a more elegant way. 
Moreover I noticed that the symbol size is kept constant when the graph is scaled (i.e. resizing the window or
changing the resolution of the output file). other software, like SigmaPlot scale also the points when a plot is
scaled.

Massimo Cressoni