Change size of axes title and labels in ggplot2

I have a really simple question, which I am struggling to find the answer to. I hoped someone here might be able to help me.

An example dataframe is presented below:

a <- c(1:10)
b <- c(10:1)
df <- data.frame(a,b)
library(ggplot2)
g = ggplot(data=df) + geom_point(aes(x=a, y=b)) +
  xlab("x axis")
g

I just want to learn how I change the text size of the axes titles and the axes labels.