This basically requires reading the help page and working with examples. It turns out that there are (at least) two different sets of attributes that can affect the size of a point. Below you will see two that I found.
require(ggplot2) require(GGally) data(diamonds, package="ggplot2") diamonds.samp <- diamonds[sample(1:dim(diamonds)[1], 200), ] # Custom Example ( almost directly from help page) pm <- ggpairs( diamonds.samp[, 1:5], mapping = ggplot2::aes(color = cut), upper = list(continuous = wrap("density", alpha = 0.5), combo = "box"), lower = list(continuous = wrap("points", alpha = 0.3, size=0.1), combo = wrap("dot", alpha = 0.4, size=0.2) ), title = "Diamonds" ) pm