You can use the character expansion factor (cex) parameter to increase or decrease the size of the text in titles, labels, axis, or subtitles.
cex: A numerical value giving the amount by which plotting text and symbols should be magnified relative to the default. The default value is 1. A value > 1 (e.g. 1.1) will increase the size and a value < 1 (e.g. 0.9) will decrease the size.
cex.axis: set the size of axis annotation relative to the current setting of cex.
cex.lab: set the size of x and y labels relative to the current setting of cex.
cex.main: set the size of main titles relative to the current setting of cex.
cex.sub: set the size of sub-titles relative to the current setting of cex.
In your code, make the following changes to increase the size by 10%.
ylab="Fraction of positives",
xlim=c(0,1),ylim=c(0,1), col="blue", main="True fraction: 0.01", cex.main=1.1, cex.lab=1.1)