par

I have a concrete example of this problem:

p <- 1:3
u <- matrix(c(1,1,1,2,2,2,3,3,3),3,)
r <- p*u
# bitmap("logq.bmp",type="bmpmono",height=3.5,width=7,res=96)
par(mfcol=c(1,3),cex=1.5,mar=c(8,4,4,0),pin=c(1.2,1.4))
matplot(p,r,type="b",main="A",col="black")
matplot(log(p),log(r),type="b",main="B",col="black")
r <- p+u
matplot(p,r,type="b",main="C",col="black")
# dev.off()

This was for someone who wanted to drop it into a Word document
(hence the lines commented out) - a test of mathematical
reasoning.  Word did not like it if the margin was too big.
(Eventually I chopped it with xv.)  Try as I might, I could not
adjust the margins so that it took up the whole space on the
page, horizontally, and the labels and figures were nicely spaced
yet not so small as to be unreadable.  If I made the margins too
small, the labels became invisible.