peewee.DataError: (1406, "Data too long for column ...
Sorry, I think this was a database issue, peewee was just passing back the error.

I'm trying to create a custom field type

```
class JSONField(Field):

    def db_value(self, value):
        return json.dumps(value)

    def python_value(self, value):
        return json.load(value)
```

And then use this to write some data. But I'm being told its too long for the column. How do I explain to peewee what size of data the column can accept?

Edit: I have also tried changing to `class JSONField(TextField):` instead of just `Field` but no change in the error.
