Being an American I do not think in metric but can usually mentally calculate a rough equivalent between English units and Metric units - thus when I estimated the diameter of this lovely mushroom as about one-half yard, and knowing that a 'yard' and a 'meter' were roughly equivalent (i.e. 39.37 inches is about a meter in length, and that 36 inches is about a yard in length) I blithely filled in a half of 39.37 inches (20") rather than a half of 100 centimeters (50 cm). As I was driving home my sub-conscious passed me a message saying that 20 cm was a little off, eh?
So with apologies, let me say that the mushroom had a diameter of about 50 cm give or take.