How to increase font size of contour labels?

Question

I am trying to increase the font size of the contour plot labels in R. Currently, the contour plot labels are so small as to be unreadable!

Setting cex does not change the font size of the contour plot labels. Which parameter should I be setting?

model <- function (a, b){ 
    23.86+5.525*b-2.5725*a-6.6413*b^2-5.1862*a^2 
} 

x <- seq(-1, 1, 0.1) 
y <- seq(-1, 1, 0.1)
z <- outer(x, y ,model)

png('contour.png', width = 1000, height = 1000)
par(cex=3)
contour(x, y, z)
dev.off()

Output:

Answer 1

Pass the labcex argument to contour with a desired scaling factor. As ?contour states, labcex is cex for contour labelling.

contour(x, y, z, labcex=5)