sizing of quartz and x11 windows

i am more wondering why 3" in the function call turns into 2 6/32"  
displayed.  is this a standard error for everyone, or is it just  
my  monitor/card-combo that creates it?

The discrepancy comes from the fact that the DPI density is not  
necessarily calibrated exactly to your monitor. I don't remember what  
Quartz uses (it is initialized to 72dpi, but I see no code that  
changes it later), but the DPI for X11 is usually set to an arbitrary  
default of 75 or 100 (see xdpyinfo|grep resol) which may not match  
your display (in fact my 23" Apple display has 99.5x98.3 DPI  
according to OS X so Apple knows what they're doing). You can use the  
-dpi argument to specify your real DPI in X11. As of Quartz, unless  
there is a way to set ipr in R directly I don't think you can change  
it. Given that most modern displays have at least 100dpi, I'd vote to  
change the Quartz default to 100...

Cheers,
Simon