The documentation for functions such as geom_line describes the parameter ... as follows (my emphasis):
Other arguments passed on to [layer()]. These are often aesthetics, used to set an aesthetic to a fixed value, like colour = "red" or size = 3. They may also be parameters to the paired geom/stat.
Now that the linetype aesthetic has replaced size for geom_line and others, this might be slightly misleading (although the relevant aesthetics are listed under the 'Aesthetics' heading). Might it be better to remove the reference to size = 3, for example?
This parameter's documentation seems to be inherited from the geom_point documentation using @inheritParams geom_point.