[O] PNG R plots size

[Vicente Vera]

Hello. I have a source code block for an R plot with the following
header:

#+BEGIN_SRC R :results output graphics :file figure1.png :exports results

It works, but the default size is too small. When I change the
extension to SVG or PDF (i.e. when exporting to LaTeX, but is not what
i need right now) the dimensions are fine.

How can I control the size of a PNG graphic? Do I need to tweak
something inside Org or specify something in R?

Thanks.