R ggplot: size of bins and labels in legend of binned variable

I want the legend of a binned variable discrete_var in the bottom of my figure. I want the bins on one row. The problem is that doing this results in different sizes of bins and the labels too close to each other. How can I set the size of bins, or rotate labels to be place under and vertical to the legend bins (which might do the trick)?

shp |>
  ggplot() +
  geom_sf(aes(fill = discrete_var)) +
  scale_fill_brewer(palette = 'Purples', direction = -1, na.value="grey", drop=FALSE) +
  coord_sf(expand = FALSE) +
  theme(legend.position = "bottom") +
  guides(fill=guide_legend(nrow=1,byrow=TRUE, label.position = "bottom"))

This is what I want, but in the bottom of the figure:

Making the legend key width wider would be one approach with legend.key.width = unit(2, "cm"):

library(tidyverse)
library(sf)
#> Linking to GEOS 3.11.2, GDAL 3.6.2, PROJ 9.2.0; sf_use_s2() is TRUE

map <- map_data("world")

mapcols <- map |>
  count(region) |>
  mutate(
    discrete_var = cut(40 * n / max(n), breaks = seq(0, 40, 5)),
    discrete_var = gsub("\\((\\d+),(\\d+)\\]", "\\1 to \\2", discrete_var)
  )

shp <-   st_as_sf(maps::map('world', plot = FALSE, fill = TRUE))

shp |> 
  left_join(mapcols, by = join_by(ID == region)) |> 
  ggplot() +
  geom_sf(aes(fill = discrete_var)) +
  scale_fill_brewer(palette = 'Purples', direction = -1, na.value="grey", drop=FALSE) +
  coord_sf(expand = FALSE) +
  theme(legend.position = "bottom", legend.key.width = unit(2, "cm")) +
  guides(fill=guide_legend(nrow=1,byrow=TRUE, label.position = "bottom"))

Or to put the text at an angle use legend.text = element_text(angle = 45, vjust = 0.5):

shp |> 
  left_join(mapcols, by = join_by(ID == region)) |> 
  ggplot() +
  geom_sf(aes(fill = discrete_var)) +
  scale_fill_brewer(palette = 'Purples', direction = -1, na.value="grey", drop=FALSE) +
  coord_sf(expand = FALSE) +
  theme(legend.position = "bottom", legend.text = element_text(angle = 45, vjust = 0.5)) +
  guides(fill=guide_legend(nrow=1,byrow=TRUE, label.position = "bottom"))

Finally, to make vertical, substitute your final line with theme(legend.position = "bottom", legend.direction = "vertical"):

shp |> 
  left_join(mapcols, by = join_by(ID == region)) |> 
  ggplot() +
  geom_sf(aes(fill = discrete_var)) +
  scale_fill_brewer(palette = 'Purples', direction = -1, na.value="grey", drop=FALSE) +
  coord_sf(expand = FALSE) +
  theme(legend.position = "bottom", legend.direction = "vertical")