code: def size(value):
          
          if value.value:
              _assert_is_type('size.value', value.value, int)
              if value.value < 0:
                  raise ValueError('size cannot be negative')
docs: ValueRef : number, area of the mark in pixels
  
          This is the total area of a symbol. For example, a value of 500 and
          a ``shape`` of ``'circle'`` would result in circles with an area of
          500 square pixels. Only used if ``type`` is ``'symbol'``.