Change size of axes title and labels in ggplot2

You can change axis text and label size with arguments axis.text= and axis.title= in function theme(). If you need, for example, change only x axis title size, then use axis.title.x=.

g+theme(axis.text=element_text(size=12),
        axis.title=element_text(size=14,face="bold"))

There is good examples about setting of different theme() parameters in ggplot2 page.

If you are creating many graphs, you could be tired of typing for each graph the lines of code controlling for the size of the titles and texts. What I typically do is creating an object (of class "theme" "gg") that defines the desired theme characteristics. You can do that at the beginning of your code.

My_Theme = theme(
  axis.title.x = element_text(size = 16),
  axis.text.x = element_text(size = 14),
  axis.title.y = element_text(size = 16))

Next, all you will have to do is adding My_Theme to your graphs.

g + My_Theme

if you have another graph, g1, just write:

g1 + My_Theme 

and so on.

I think a better way to do this is to change the base_size argument. It will increase the text sizes consistently.

g + theme_grey(base_size = 22)

As seen here.