change default output size when using Sweave

I am trying to create a graphic output in Sweave but I do not want it to be
standard size. I want the whole graphic to be 80mm of height only, just like
the viewport below.

<<fig=TRUE, echo=TRUE>>=
   pushViewport(viewport(height = unit(80, "mm")))    
   grid.rect()
   grid.text("I want this viewport to be the whole output size")
   popViewport()
@

How can I make the graphic smaller (80mm of height) without having the white
margins at the top and bottom?

You can specify height and width as arguments to the code chunk or set 
new defaults with something like

\SweaveOpts{width=5,height=3}

(I think those measurements need to be given in inches; I don't think 
there's a way to use an expression like unit(80,"mm").)

You can also control the size of the plot in the final document with 
LaTeX code like

\usepackage{graphicx}
\setkeys{Gin}{width=3in}

and then the LaTeX rules for handling units would apply, so 80mm is fine 
instead of 3in.

Duncan Murdoch