Hollow circles for size legend

guides.md

library(tidyverse)

set.seed(23)

d <- tibble(x = rnorm(10), y = rnorm(10), a = rep(c("x", "y"),5), b = rep(1:5, 2))

d %>% 
  ggplot(aes(x, y, color = a, size = b)) + geom_point() +
  guides(colour = guide_legend(order = 1), 
         size = guide_legend(order = 2, override.aes = list(shape = 21, stroke = 0.5)))

^{Created on 2021-06-30 by the reprex package (v2.0.0)}