increase legend font size ggplot2

theme(plot.title = element_text(size = 12, face = "bold"),
    legend.title=element_text(size=10), 
    legend.text=element_text(size=9))

You can use theme_get() to display the possible options for theme. You can control the legend font size using:

+ theme(legend.text=element_text(size=X))

replacing X with the desired size.