Took me too long to figure this out...

plot-from-var.R

library(ggplot2)

df = data.frame(foo = rep(c("foo", "bar"), each = 50),
                bar = c(rep("tom", 33),
                        rep("harry", 33),
                        rep("dick", 34)),
                big = runif(n = 100),
                small = rnorm(n = 100))

testPlot = function(df, var1, var2) {
  ggplot(df,
         aes_string(x = "big",
                    y = "small",
                    colour = var1,
                    size = var2)) +
    geom_point()
}

testPlot(df = df,
         var1 = "foo",
         var2 = "bar")