code: def sizes(self):
          
          json = self.request('/sizes', method='GET')
          status = json.get('status')
          if status == 'OK':
              sizes_json = json.get('sizes', [])
              sizes = [Size.from_json(s) for s in sizes_json]
              return sizes
          else:
              message = json.get('message')
              raise DOPException('[%s]: %s' % (status, message))
docs: This method returns all the available sizes that can be used to create
          a droplet.