I think the answer is 24.
If we see, there will be only three elements after all the operations which may prompt us to think the size of the stack is =12 bytes (3*4 bytes) .
But STACK is a static data structure and hence needs to allocate space for all the numbers even if they are not present in the stack simultaneouly.
Here , numbers are - 5 5 6 6 7 8 (Consider them as block of memory rather than nummbers )
So, size of the datastructure will be = 6* 4 Bytes= 24 Bytes
PUSH and POP operations doesn't allocate or free memory . They are simply the Increment/decrement of Stack Pointer.