ggplot2 - The unit of size

The answer is : The unit is the points. It is the unit of fontsize in the grid package. In ?unit, we find the following definition

"points" Points. There are 72.27 points per inch.

(but note the closely related "bigpts" Big Points. 72 bp = 1 in.)

Internally ggplot2 will multiply the font size by a magic number ggplot2:::.pt, defined as 1/0.352777778.

Here a demonstration, I create a letter using grid and ggplot2 with same size:

library(grid)
library(ggplot2)
ggplot(data=data.frame(x=1,y=1,label=c('A'))) +
  geom_text(aes(x,y,label=label),size=100)
## I divide by the magic number to get the same size.
grid.text('A',gp=gpar(fontsize=100/0.352777778,col='red'))

Addendum Thanks to @baptiste

The "magic number"(defined in aaa-constants.r as .pt <- 1 / 0.352777778) is really just the conversion factor between "points" and "mm", that is 1/72 * 25.4 = 0.352777778. Unfortunately, grid makes the subtle distinction between "pts" and "bigpts", which explains why convertUnit(unit(1, "pt"), "mm", valueOnly=TRUE) gives the slightly different value of 0.3514598.

The 'ggplot2' package, like 'lattice' before it, is built on the grid package. You can get the available units at:

?grid::unit
?grid::convertX
?grid::convertY

grid::convertX(grid::unit(72.27, "points"), "inches")

(I use the formalism pkg::func because in most cases grid is loaded a a NAMESPACE but not attached when either lattice or `ggplot2 are loaded.)

I earlier posted a comment that I later deleted saying that size was in points. I did so after seeing that the size of the text with size=10 was roughly 10 mm. The "magic" number mentioned by agstudy is in fact within 1% of:

as.numeric(grid::convertX(grid::unit(1, "points"), "mm"))
#[1] 0.3514598
0.352777778/.Last.value
#[1] 1.00375

From ?aes_linetype_size_shape

# Size examples
# Should be specified with a numerical value (in millimetres),
# or from a variable source

height and width in ggsave relate to par("din") from ?par

din

  R.O.; the device dimensions, (width, height), in inches. See also dev.size,
  which is updated immediately when an on-screen device windows is re-sized.

So I guess size in aes is in millimetres and ggsave height and width are in inches.