The Size item in the Call Stack window is the size, in bytes, of the stack frame for the called function. A function's stack frame includes the address to return to when the function completes, a pointer to the previous function's stack frame, and any local variables declared in the calling function. Given that the return address and previous function pointer are fixed-size components of a stack frame, the stack frame size provides an indication of how much local data is declared by the function.
In C++, because the heap, not the stack frame, contains the storage for dynamically allocated local variables, only the pointers to such variables take up stack frame space.
Select Size from the Options->Display style dialog for the Call Stack window if you want a column showing the size of the stack frame.