[R] How to set the size of a rgl window, par3d() ?

[begert]

Dear R- Users,

   is there a way to determine the size of
   an rgl window (rgl.open()) either in advance or
   afterwards, (without using the mouse, of course) ?

   Intuitively, one would assume to set the size by:

   library("rgl");
   par3d(viewport=c(0,0,500,500));
   #rgl.open();

   for example. As the parameter 'viewport' is 'readonly'
   this results in an error message:
   Error in par3d(viewport = c(0, 0, 500, 500)) :
   invalid value specified for rgl parameter "viewport"
   In addition: Warning message:
   parameter "viewport" cannot be set.

   Any possible workarounds ?

Thanks
Bjoern