What is the maximum size of the stack?

On Windows, the typical maximum size for a stack is 1MB, whereas it is 8MB on a typical modern Linux, although those values are adjustable in various ways.

Why is stack size limit?

The maximum stack size is static because that is the definition of “maximum”. Any sort of maximum on anything is a fixed, agreed-upon limiting figure. If it behaves as a spontaneously moving target, it isn’t a maximum. Stacks on virtual-memory operating systems do in fact grow dynamically, up to the maximum.