I have a panel containing three plots. How can I use par to specify the width and height of the main panel so it is always at a fixed size?
par
You do that in the device, e.g.
x11(width=4, height=6)
and similarly for the file-based ones
pdf("/tmp/foo.pdf", width=4, height=6)
You can read the physical size via par("cin") etc but not set it.
par("cin")