On library V1.x.x : The stack size is fixed in the startup_gcc.c at 80K (20K * 4 bytes (32 bitwords)). It starts at address 0x0.
In V2.x it is more complex. In the file, AMA3B1KK.ld defines that the STACK is counting down. The stack size pointer starts at the top 384k - 8k and grows downwards. This is the TOTAL stack size. Now there are many different stack sizes in MBED e.g. for each thread that is running 4K. Look in mbed_config.h (in the variants/"variant"/mbed folder) Be aware however that the "heap-space" starts above the program memory .bss space. If you do a malloc() the heap will grow upwards. Potentially heap-growing-up and stack-growing-down could collide causing a hard failure. To change stack sizes in MBED, there are many articles on the web.