What is the stack size in Linux?

The stack size, referes to how much space is allocated in memory for the stack. If you increase the stack size, that allows the program to increase the number of routines that can be called. Each time a function is called, data can be added to the stack (stacked on top of the last routines data.)

How is stack size determined?

You should see garbage for the part of the stack that has been used and the “STACK—” strings in the remainder of the stack. Count the number of complete strings, multiply by 8 (since “STACK—” is 8 bytes long), and you have the number of bytes of remaining stack space.