There isn't any difference Ren. 'toadstool' just comes from the german 'todes stuhl' or 'death's seat'. And there is no botanical separate class, its just a colloquial term that people use.
And there certainly are edible kinds that can grow that big (or bigger in some cases, giant puffball for instance can end up weighing as much as either of us, and a meter and a half in diameter, although it doesn't have the traditional 'mushroom' shape. Just a big white, usually irregular shaped ball of tasty.) I've found plenty that are absolutely huge, though when it comes to normal mushrooms. Parasols in particular have a tendency to grow huge caps [Macrolepiota procera] and fried in salted butter they are truly delicious. And some of the wild Agaricus especially, relatives of the common shop button mushrooms, some of them, like A.macrosporus can grow REALLY big. Not just wide but squat and generally hefty, thick things, as well as the one known as the prince, A.augustus, can grow to over 20 inches in diameter (although the prince has a much thinner, taller stem than A.macrosporus.) I've found A.macrosporus before, having a faint, delicate taste and smell of aniseed under its typically 'mushroomy' smell and taste, and they have been really heavy, stocky things that need to be sliced to be cooked properly because they are otherwise too thick and hefty to cook through and through easily.
And some of the edible boletes can grow to be a really considerable size, as can some Suillus, a relative genus to the boletes proper, like the slippery jack, those can easily grow to be of a size that would take a side-plate to hold one large cap. (very tasty too, I love slippery jacks, once the slime layer on the cap is peeled away, and they are cooked in garlic butter and given a squeeze of lemon before eating, one of my favourites actually), Boletus/Xerocomus badius, the bay bolete, can grow to a similarly huge size, and again is very good eating.
Found plenty of saffron milkcaps last year too that were almost the size of a side-plate, (Lactarius deliciosus, the 'milk cap' name for the Lactarius genus comes from the fact that when the flesh is broken or cut, then there is the exudation of a milky, sap-like liquid, that varies both in color, and in some kinds, the way, if any, that after exposure to air, the milk changes color, in the case of the saffron milk-cap, the entire thing is bright orange, and so is the milk, aside from when bruised, a slight greenish tinge develops, although that is more prominent in the quite similar, although more bitter and whilst still good eating, not quite so desirable L.deterrimus), and L.deliciosus more than lives up to its species epithet, meaning 'delicious', no surprise there. Very good fried, even better roasted and given a topping of mozzarella cheese over the gills with a little salt and sprinkling of black pepper, then grilled to melt the cheese until it bubbles up and goes nice and golden brown on the top. They aren't too common, but I do have one spot for them in a pine and mixed deciduous forest that tends every year to fruit L.deliciosus very prolifically indeed, along with plenty of various boletes, and both slippery jack and the similar, and just as tasty larch bolete/larch Suillus. Ceps, and bay boletes too, tend to fruit in abundance. And for some reason right near the car park, I almost always find just one, sometimes two individuals of Lepiota brunneo-incarnata, a quite rare species, although not one to be eaten, as it is full of the same deadly amatoxins as the notorious death cap (Amanita phalloides) and destroying angel (A.virosa, plus some of its relatives within the subsection Phalloidea of the genus Amanita, the group which contain those deadly white-spored Amanita species that produce amatoxins, and worldwide cause 90% of fatalities associated with people eating the wrong mushroom. Nasty little bastards that slowly destroy the liver via inhibition of RNA polymerase III and binding to the actin fibers that make up a large portion of the cytoskeleton, as well as targeting kidneys, fucking with the electrolyte levels in the blood and sometimes damaging the heart, usually if not treated both very quickly and aggressively, resulting in a very, very unpleasant and drawn out death for the poor unfortunate bastard to have eaten them, with even a single bite of a death cap or its relatives being quite enough to kill the biter.
But for some reason, although its usually just the one single fruit-body, or just occasionally I'll see two of them, I tend to find these rare Lepiotas just on the entrance to the forest proper at the end of the car-park/start of the forest paths. Year after year, trip after trip, they seem to be there, close to where I'd have seen it the last year I went. Needless to say, those are most certainly staying off the menu of the mushrooms and wild bilberries I eat from there, not like theres any shortage of very good wild edibles from that particular forest. Big shopping bags laden with saffron milk-caps alone, and thats without even considering the large haul of boletes I find most trips I take to the place.
