ggplot2: Adjust the symbol size in legends - r

Question

How should I change the size of symbols in legends? I checked the document of theme but found no answer.
Here is an example:
library(ggplot2);library(grid)
set.seed(1000)
x <- 1:6
mu <- sin(x)
observed <- mu + rnorm(length(x), 0, 0.5*sd(mu))
data <- data.frame(
t=rep(x, 2),
value=c(mu, observed) - min(mu, observed) + 0.5,
class = rep(c("mu", "observed"), each=length(x)))
mu <- data$value[1:length(x)]
observed <- data$value[1:length(x) + length(x)]
mu.min <- mu - 3 * 0.5 * sd(mu)
mu.max <- mu + 3 * 0.5 * sd(mu)
g <- ggplot(data=data)
g <- g + geom_point(aes(x=value, y=t, shape=class, size=24)) + scale_size(guide="none")
g <- g + scale_shape(name="", labels=expression(paste(S[u](t), ", the observation at time ", t), paste(mu[u](t), ", the mean of ", tilde(S)[u](t), " ")))
stat_function.color <- gray(0.5)
g <- g + geom_segment(aes(y=1:6, yend=1:6, x=mu.min, xend=mu.max, linetype="2", alpha = 1), color=stat_function.color) + scale_alpha(guide="none") + scale_linetype(name= "", labels=expression(paste("probability density function (pdf) of ", tilde(S)[u], " at time ", t)))
for(i in 1:length(x)) {
g <- g + stat_function(fun=function(x, i) {
ifelse( x <= mu.max[i] & x >= mu.min[i], dnorm(x, mu[i], sd(mu)) + i, NA)
}, color=stat_function.color, args=list(i=i))
}
background.color <- gray(0.75)
g <- g + theme(
axis.text=element_blank(),
title=element_text(size=rel(1.5)),
legend.text=element_text(size=rel(1.5)),
legend.position="top",
legend.direction="vertical",
# legend.key.size = unit(2, "cm"),
panel.background=element_rect(fill=background.color),
panel.grid.major=element_line(color=background.color),
panel.grid.minor=element_line(color=background.color)
) + coord_flip()
plot(g)

Answer 1

You should use:
theme(legend.key.size = unit(3,"line"))

Answer 2

You can make these kinds of changes manually using the override.aes argument to guide_legend():
g <- g + guides(shape = guide_legend(override.aes = list(size = 5)))
print(g)

Answer 3

Marius's answer did not work for me as of R version 3.2.2. You can still call guide_legend() with the same override.aes argument but you will need to specify color instead of shape in the wrapper function.
So if you're running a later version of R, try this instead:
g + guides(color = guide_legend(override.aes = list(size=5)))
EDIT
As pointed out by #Ibo in the comment, this may have been due to the color scale in the ggplot object. If the ggplot object contains a color scale, the mapping of size (size=5) has to be set on the color instead.

Answer 4

If you want to change the sizes of 2 components of a legend independently, it gets trickier, but it can be done by manually editing the individual components of the plot using the grid package.
Example based on this SO answer:
set.seed(1)
dat <- data.frame(x = runif(n = 100),
x2 = factor(rep(c('first', 'second'), each = 50)))
set.seed(1)
dat$y = 5 + 1.8 * as.numeric(dat$x2) + .3 * dat$x + rnorm(100)
# basic plot
g <- ggplot(data = dat,
aes(x = x, y = y, color = x2))+
theme_bw()+
geom_point()+
geom_smooth(method = 'lm')
# make the size of the points & lines in the legend larger
g + guides(color = guide_legend(override.aes = list(size = 2)))
# Make JUST the legend points larger without changing the size of the legend lines:
# To get a list of the names of all the grobs in the ggplot
g
grid::grid.ls(grid::grid.force())
# Set the size of the point in the legend to 2 mm
grid::grid.gedit("key-[-0-9]-1-1", size = unit(4, "mm"))
# save the modified plot to an object
g2 <- grid::grid.grab()
ggsave(g2, filename = 'g2.png')

Answer 5

As of 12/1/2022, in ggplot2 version 3.4.0, the argument:
guides(shape = guide_legend(override.aes = list(size = 5)))
no longer works....instead, replace "size = 5" with "linewidth = 5"