I believe you are using the module Field Formatter Class to have this option for your fields.
The module does not set a maximum value for this field. So I believe it is using a default of 128 chars in core.
128 is a fairly long limit for a class field. But if you want to alter this you can write an alter function in your module like they have done in their module and set the #maxlength property of the field_formatter_class element.
Having said that, the following is a quick hack to get this working for you.
You can open the file field_formatter_class/field_formatter_class.module
and add a line
'#maxlength' => 256,
in the element class array in the alter function in line 44. After modification this will be like the following (view original code)
function field_formatter_class_field_formatter_settings_form_alter(&$settings_form, $context) {
$display = $context['instance']['display'][$context['view_mode']];
$settings = $display['settings'];
$settings_form['field_formatter_class'] = array(
'#type' => 'textfield',
'#title' => t('Field Formatter Class'),
'#maxlength' => 256,
'#default_value' => $settings['field_formatter_class'],
);
Do note that this is modifying a contrib module and is generally not a recommended approach. You will have to maintain this patch in a patches folder.