title: ggsave png error with larger size
body: <p>I'm saving a faceted ggplot2 plot which works fine to save at a smaller size, but fails when I want to increase it.</p>
  
  <pre><code>&gt; ggsave("tst.png",height=6.75,width=9)
  # works fine
  
  &gt; ggsave("tst.png",height=9,width=12)
  Error in grDevices::png(..., width = width, height = height, res = dpi,  : 
    unable to start device
  In addition: Warning messages:
  1: In grDevices::png(..., width = width, height = height, res = dpi,  :
    Unable to allocate bitmap
  2: In grDevices::png(..., width = width, height = height, res = dpi,  :
    opening device failed
  </code></pre>
  
  <p>I've saved pngs of this size before with ggsave, any ideas why its not working?</p>
  
  <p><strong>Reproducible example:</strong></p>
  
  <pre><code>library(car)
  qplot(education,data=Vocab,geom="density",colour=sex)+facet_wrap(~year)
  </code></pre>