Legend

Remove the legend alltogether:

ggplot(data=iris,
       mapping=aes(x=Sepal.Length,
                   y=Petal.Length, color=Species))+
  ggtitle("Iris")+
  geom_point()+
  gg_("remove legend")

Change legend font size:

ggplot(data=iris,
       mapping=aes(x=Sepal.Length,
                   y=Petal.Length, color=Species))+
  ggtitle("Iris")+
  geom_point()+
  gg_("set font size of legend title to 40")