scale_size_area(..., max_size = 6)
continuous_scaleto control name, limits, breaks, labels and so forth.
scale_size_area is used, the default behavior
is to scale the area of points to be proportional to the
Note that this controls the size scale, so it will also control the thickness of lines. Line thickness will be proportional to the square root of the value, which is probably undesirable in most cases.