pthreads doesn't have a notion of maximum stack size. So there's no way to ask pthread to give a thread the "maximum allowable" stack size. There's also no way to ask pthread for a recommended maximum allowable stack size for a known set of threads.
What a suitable (maximum?) stack size is for your particular application depends on so many things: machine, OS, available memory, expected load, expected stack consumption etc., so it's really no wonder pthreads can't tell you. I can't either.
You'll have to figure out, somehow, what the proper stack size is yourself.