[R] change default output size when using Sweave

[Mark Heckmann]

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?

A lot of thanks in advance,
Mark