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.
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.