16 x 12 inches
Those inexperienced with tonal painting, would probably be afraid to use the very dark tones seen in the figures (the arms and shadows in the blond hair are almost black) but they are critical to producing, through contrast, the effect of brilliant light reflecting off the foam.
It's a useful exercise to look at the different tones in an image in isolation from each other. You'll be surprisied, maybe even shocked, at how much darker or lighter the isolated tones really are compared to their appearance in the whole image. You can use a piece of opaque paper with a small hole in it, or, with digital images, use the eye-dropper tool in PhotoShop to sample tones and hues.