Awesome, thank you for the feedback, Chris!

A mountain sized mushroom can always only exist one at a time (when triggered) and only for a few hours, after they will transform back into their original form. All mushrooms are connected through a Mycelium link, that can be physical but is mostly a non-physical energy network on a different plane of existence. An overly large mushroom would be just as large as a Stone Golem (no idea how tall Golems are but I assume like 2-3 meters). Most mushrooms are smaller than that and around the size of men. Some are barely visible spores and micro-mushrooms.

I would love to see some maps and learn more about the mushroom lore you already have.

And yes, they are clearly ancient creatures who precede most other life, even though they have been long forgotten and survived only in myths and fables, until they finally reappeared.