Where (or how) do players activate the Wild Wasteland perk...? Is it offered on a certain level...? It sounds zany and fun...!

If you choose this trait at the beginning you can get different random encouters. You don't have to activate it. But be aware this is not for the faint of heart. Hope this is the awnser your looking for.

It is one of the traits you can pick at the start of the game, in your conversation with Doc Mitchell in Goodsprings, right after you pick your SPECIAL stats and all.

What exactly does it do and what does mean ?

does it make enemy mobs larger / harder to kill ?

is it the cause of the legion assassins spawning everywhere i know there to be a bed when i sorta need 1 ?

You can get it in the start,or get it assigned by the auto doc in the Sink and big mountain.