ggplot2: how to change the width of colour aesthetic in a bar/col geometry?

I am making a column/barplot, where I change the fill aesthetic (inner colour of bar) with one variable, and the colour aesthetic (colour around the bar) with another variable. The colour around the bar is not easy to see, so I would like to increase the width of that bar. How can I do that? I don't see any width argument in scale_color_manual?

library(ggplot2)

dat <- data.frame(x = 1:5, y = 1:5, p = 1:5, q = factor(1:5),
                  r = factor(1:5))
data <-  data_frame(x = rep("a", 4),
                    y = c(1,2,4,3),
                    categ = c("1", "2", "3", "4"),
                    group = rep(c("a", "b"), each=2))

ggplot(data, aes(x, y, fill= categ, colour=group)) + 
  geom_col() +
  scale_color_manual(values = c("red", "blue")) 

Thanks!