Like _SimpleSizer except for a fixed-size field.  The input is the size
  of one value.

def _FixedSizer(value_size):
  """Like _SimpleSizer except for a fixed-size field.  The input is the size
  of one value."""

  def SpecificSizer(field_number, is_repeated, is_packed):
    tag_size = _TagSize(field_number)
    if is_packed:
      local_VarintSize = _VarintSize
      def PackedFieldSize(value):
        result = len(value) * value_size
        return result + local_VarintSize(result) + tag_size
      return PackedFieldSize
    elif is_repeated:
      element_size = value_size + tag_size
      def RepeatedFieldSize(value):
        return len(value) * element_size
      return RepeatedFieldSize
    else:
      field_size = value_size + tag_size
      def FieldSize(value):
        return field_size
      return FieldSize

  return SpecificSizer