Plugin fieldtype settings for ColumnType::LongText being inserted as varchar(255)
The Field Types defineSettings method defines the settings that get handled when you create the field. Those field settings get stored as json in the craft_fields.settings column when the field is created.

The BaseFieldType class has another method you can add to your class called defineContentAttribute. In your case, getting the column to be the data type you want would look like:

public function defineContentAttribute()
{
    return [
        AttributeType::String,
        'column'  =&gt;  ColumnType::LongText
    ];
}


The PlainTextFieldType and RichTextFieldType classes have similar use cases which modify the default defineContentAttribute value if you want to take a look at a few more examples.