You want something like the cex=1.5 argument to scale fonts 150 percent. But do see help(par) as there are also cex.lab, cex.axis, ...

Thus, to summarise the existing discussion, adding

cex.lab=1.5, cex.axis=1.5, cex.main=1.5, cex.sub=1.5

to your plot, where 1.5 could be 2, 3, etc. and a value of 1 is the default will increase the font size.

x <- rnorm(100)

cex doesn't change things

hist(x, xlim=range(x),     xlab= "Variable Lable", ylab="density", main="Title of plot", prob=TRUE)hist(x, xlim=range(x),     xlab= "Variable Lable", ylab="density", main="Title of plot", prob=TRUE,      cex=1.5)

Add cex.lab=1.5, cex.axis=1.5, cex.main=1.5, cex.sub=1.5

hist(x, xlim=range(x),     xlab= "Variable Lable", ylab="density", main="Title of plot", prob=TRUE,      cex.lab=1.5, cex.axis=1.5, cex.main=1.5, cex.sub=1.5)

Notice that "cex" does change things when the plot is made with text. For example, the plot of an agglomerative hierarchical clustering:

library(cluster)data(votes.repub)agn1 <- agnes(votes.repub, metric = "manhattan", stand = TRUE)plot(agn1, which.plots=2)

will produce a plot with normal sized text:

and plot(agn1, which.plots=2, cex=0.5) will produce this one: