I've done some more testing, and the thread stack size could be smaller than 16MB, but definitely not as small as the value that's currently in thread_pthread.h.

I prefer to have a slightly too large value here because IIRC the relation between the recursion limit and C stack usage is a heuristic. 

BTW. It should be possible to implement PyOS_CheckStack() on macOS. There are (undocumented) APIs in the public header files to get the stack base and size for the current thread. These APIs also work for the main thread:

# ---

#include <pthread.h>
#include <stdio.h>

int main(void)
{
        printf("%#lx\n", (long)pthread_get_stacksize_np(pthread_self()));
        printf("%p\n", pthread_get_stackaddr_np(pthread_self()));
}

# ---