measurement unit

Dear R-list,

Could anybody tell me where to find information about changing the measurement 
unit from inch to centimeters? 
I read the help from X11, I read R-intro and I did some searhing in the R 
archives, but I couldn't find the answer.
For example, I would like to produce a plot of a certain width and height:

X11(width=10, height=5)

and I would like these to be centimeters, rather than inches.

Thank you,
Adrian

Adrian Dusa
Romanian Social Data Archive
1, Schitu Magureanu Bd
050025 Bucharest
Romania
Tel./Fax: +40 21 3126618 \
              +40 21 3120210 / int.101