The prolog block has a header and a footer and an empty payload and so the size of the prolog block is 8 16.

The prolog block is marked as allocated.

So in binary, the header is

        0000 0000 0000 0000 .... 0000 0000 0001 0001
     

As an integer, this looks like 17, but the lower order 4 bits are not part of the size. The size has 4 implied (not stored) low order bits equal to 0:

        0000 0000 0000 0000 0000 0000 00010000
     

which is the value 16

It is convenient to write the header value as 16/1: size 16 and allocated bit = 1 or 16/0 for a free block of size 16..