Official comment
In cases where the Field Editor has warned about data limits, you can convert your text fields to long text. Because the long text field can hold at least 20,000 characters you will not lose data by changing the field type. The long text field is a pointer to an external record with the text in it, saving space in the database. Because the data is fetched from another database there is a performance penalty, and we don't recommend trying to add hundreds of long text records to your database.
You can convert a text field to long text by simply editing the field. Note that you cannot convert the field back to a regular text field and a long text field cannot be used as a column heading in a list view.

As an alternative, you can have Support submit a ticket to convert all your custom text fields to long text.
ONTRAPORT can also trim fields, shortening the 256 character limit in text fields to a shorter length, such as 150 characters. Note that data in these fields are truncated at the trimmed point, losing the data after that point in the field.
There is a bit more in our Knowledge Base article at http://support.ontraport.com/entries/...