This is my model:
class Position(models.Model): map = models.ForeignKey(Map,primary_key=True) #members=models.CharField(max_length=200) LatLng = mode
That depends on the database backend. The Django DB Docs will tell you, that max_length=255 is guaranteed to work always.
max_length=255
If you need something of the amount you've specified in your question, I'd suggest to use a TextField.