Imagine I want to find the biggest Amanita muscaria in the world. For this queeste I want to use an observation field in which I can register the diameter of the cap
Well, what do the numbers tend to say? If it’s things like “3” and “5”, that’s definitely in inches, as there are no 5-meter mushroom caps. If it’s things like “.08”, that’s probably in meters.
Mushrooms are fruiting bodies of fungi, so not necessary a cap-and-stem arrangement. Toadstool refers to cap-and-stem (though some have gills, like Amanitas, some have pores like Boletus).
A precondition of finding the largest Amanita would also be that observers have measured and recorded cap diameter - which most of them haven’t.