How to change the default font size in ggplot2

Use theme_set if you want to update for the remainder of your active session:

theme_set(theme_grey(base_size = 18)) 

If you only want to change one graph you can set the base_size in the theme:

qplot(1:10, 1:10) + theme_grey(base_size = 18) 
ggplot(mtcars, aes(x = mpg, y = cyl)) + 
geom_point() +
theme_grey(base_size = 18) 

Use theme_set()

theme_set(theme_gray(base_size = 18))
qplot(1:10, 1:10)