Try the cex argument:
?par
cex
A numerical value giving the
amount by which plotting text and
symbols should be magnified relative
to the default. Note that some
graphics functions such as
plot.default have an argument of this
name which multiplies this graphical
parameter, and some functions such as
points accept a vector of values
which are recycled. Other uses will
take just the first value if a vector
of length greater than one is
supplied.