The easiest way I know of is to set the base_size in a theme call. This is done within the ggplot, so no additional dependency is needed.
But it has to be done for each plot individually, there is no global option that I know of.
library(tidyverse)
ggplot(mtcars, aes(wt, mpg)) +
geom_point() +
labs(title = "Fuel economy declines as weight increases") +
theme_classic(base_size = 25) # big, big text