How much room should u give fungi to grow?

All of it.
But really they don't need much room. They tend to be very resilient. Give 6 inches past the expected maximum growth. So if it's gonna be 5 inches tall, and 3 inches wide, make it a 9x9x11 area. And that's being generous

Depends on the fungus.

https://www.sciencemag.org/news/2018/10/humongous-fungus-almost-big-mall-america

i see what you did there..