Use dev.new(). (See this related question.)
plot(1:10)
dev.new(width=5, height=4)
plot(1:20)
To be more specific which units are used:
dev.new(width=5, height=4, unit="in")
plot(1:20)
dev.new(width = 550, height = 330, unit = "px")
plot(1:15)
edit additional argument for Rstudio (May 2020), (thanks user Soren Havelund Welling)
For Rstudio, add dev.new(width=5,height=4,noRStudioGD = TRUE)
This will depend on the device you're using. If you're using a pdf device, you can do this:
pdf( "mygraph.pdf", width = 11, height = 8 )
plot( x, y )
You can then divide up the space in the pdf using the mfrow parameter like this:
par( mfrow = c(2,2) )
That makes a pdf with four panels available for plotting. Unfortunately, some of the devices take different units than others. For example, I think that X11 uses pixels, while I'm certain that pdf uses inches. If you'd just like to create several devices and plot different things to them, you can use dev.new(), dev.list(), and dev.next().
Other devices that might be useful include:
There's a list of all of the devices here.
A convenient function for saving plots is ggsave(), which can automatically guess the device type based on the file extension, and smooths over differences between devices. You save with a certain size and units like this:
ggsave("mtcars.png", width = 20, height = 20, units = "cm")
In R markdown, figure size can be specified by chunk:
```{r, fig.width=6, fig.height=4}
plot(1:5)
```
As the accepted solution of @Shane is not supported in RStudio (see here) as of now (Sep 2015), I would like to add an advice to @James Thompson answer regarding workflow:
If you use SumatraPDF as viewer you do not need to close the PDF file before making changes to it. Sumatra does not put a opened file in read-only and thus does not prevent it from being overwritten. Therefore, once you opened your PDF file with Sumatra, changes out of RStudio (or any other R IDE) are immediately displayed in Sumatra.
Source: Stackoverflow.com