Topic: How much tmpfs space will I need?

Let's say I am using a computer that has only 8GB of RAM, like the Thinkpad T400s.
I am planning to mount all these directories as tmpfs filesystems to ensure I reduce SSD wear and tear as much as possible.

(I've been doing my research over what directories to mount to my RAMdisk)

/dev (devtmpfs)
/var/tmp (Assuming Hyperbola Linux and BSD have folder2ram)

Basically, I'm asking how much RAM space will I need for each of these directories if I have only 8GB RAM?

If I have any more, like as much as 128GB, can I fare any better with bigger tmpfs capacities?