[racket] Controlling the size of plot symbols

Hi everyone,

I am trying to use (abuse?) Racket's 3D plots for molecular
visualization.  It actually works out better than I expected, except
that I haven't yet figured out how to control the size of the plot
symbols in the way I want.

The documentation says no more than that the size of a symbol is a
positive number. But how is this number interpreted?

What I would like to have is "full circles" whose radii are equal to
the atomic radii of my atoms. That means I have to set the symbol size
on the same scale as the coordinates. My experiments make me suspect
that symbol size is interpreted as an absolute size in pixels or
whatever. Is it somehow possible to change this?

Thanks in advance,
  Konrad.