R: ggplot2, can I set the plot title to wrap around and shrink the text to fit the plot?

You have to manually choose the number of characters to wrap at, but the combination of strwrap and paste will do what you want.

wrapper <- function(x, ...) 
{
  paste(strwrap(x, ...), collapse = "\n")
}

my_title <- "This is a really long title of a plot that I want to nicely wrap and fit onto the plot without having to manually add the backslash n, but at the moment it does not"
r + 
  geom_smooth() + 
  ggtitle(wrapper(my_title, width = 20))

I do not think there is a text wrap option in ggplot2 (I have always just inserted \n manually). You can, however, shrink the size of the title's text by altering your code in the following way:

title.size<-10
r + geom_smooth() + opts(title = my_title,plot.title=theme_text(size=title.size))

In fact, you all aspects of text with the theme_text function.

Just for an update as has been mentioned in the comments opts is deprecated. You need to use labs and you could do:

library(ggplot2)

my_title = "This is a really long title of a plot that I want to nicely wrap \n and fit onto the plot without having to manually add the backslash n, but at the moment it does not"

Option 1: Using str_wrap option from the stringr package and setting your ideal width:

 library(stringr)
 ggplot(data = cars, aes(x = speed, y = dist)) +
      geom_smooth() +
      labs(title = str_wrap(my_title, 60))

Option 2: Using the function provided by @Richie https://stackoverflow.com/a/3935429/4767610 like this:

wrapper <- function(x, ...) 
{
  paste(strwrap(x, ...), collapse = "\n")
}
ggplot(data = cars, aes(x = speed, y = dist)) +
      geom_smooth() +
      labs(title = wrapper(my_title, 60))

Option 3: Using the manual option (granted, this is what the OP wanted to avoid but it might be handy)

my_title_manual = "This is a really long title of a plot that I want to nicely wrap \n and fit onto the plot without having to manually add \n the backslash n, but at the moment it does not"

 ggplot(data = cars, aes(x = speed, y = dist)) +
          geom_smooth() +
          labs(title = my_title_manual)

Option 4: Reduce the text size of the title (as in the accepted answer https://stackoverflow.com/a/2633773/4767610)

ggplot(data = cars, aes(x = speed, y = dist)) +
  geom_smooth() +
  labs(title = my_title) +
  theme(plot.title = element_text(size = 10))