R support - exporting high res. plots

Hi Guys, I have just been hitting my head against a brink wall trying to figure out he best way to export ggplot2 graphs in 600 dpi .tiff format for publication. This may be useful for many of you. The code is

ppi = 600
tiff("FIG1.tiff", width=6*ppi, height=8*ppi, res=ppi)
grid.arrange(fig_a, fig_b, fig_c, ncol=1, nrow=3)
dev.off()

This is the post that helped me get there;
http://stackoverflow.com/questions/9771619/how-to-increase-the-size-of-the-graph-while-maintaining-the-relative-scale-of-it/9771817#9771817

For all the R whizz kids out there, please respond to this comment if you have advice on how to do this.

Merci,
Cécile