code: func (f *freelist) size() int {
  	n := f.count()
  	if n >= 0xFFFF {
  		// The first element will be used to store the count. See freelist.write.
  		n++
  	}
  	return pageHeaderSize + (int(unsafe.Sizeof(pgid(0))) * n)
  }
docs: // size returns the size of the page after serialization.