How to modify ggplot2 legend keys?

Question

Is there a way to change the width and height of the keys in the legend with ggplot2? In the following example, I would like to replace the dots in the legend with rectangles that I could adjust the width and height. I have tried to use keywidth without success.

library(ggplot2)

ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
  geom_point() +
  theme(
    legend.position = "top",
    legend.title = element_blank()
  ) +
  guides(
    color = guide_legend(
      label.position = "top",
      override.aes = list(shape = 15, size = 5),
      keywidth = unit(2, "cm") # This is not giving me what I was expecting.
    )
  )

^{Created on 2020-07-23 by the reprex package (v0.3.0)}

Answer 1

While @Ian's answer works, there's a far simpler way, which is to define the legend key glyph you want to use right in the geom_point() call. The important point to note is that if we specify the key glyph should be a rect, we need to provide the fill aesthetic (or you'll just have empty rectangles for the glyphs):

ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
  geom_point(aes(fill=Species), key_glyph='rect') +
  theme(
    legend.position = "top",
    legend.title = element_blank()
  )

You should be able to adjust the key dimensions from there via guides() or theme() changes to suit your needs.