ValidatorFn maxLength(num maxLength)

Validator that requires controls to have a value of a maximum length.

Source 

static ValidatorFn maxLength(num maxLength) {
  return /* Map < String , dynamic > */ (model_module
      .AbstractControl control) {
    if (Validators.required(control) != null) return null;
    String v = control.value;
    return v.length > maxLength
        ? {
            "maxlength": {
              "requiredLength": maxLength,
              "actualLength": v.length
            }
          }
        : null;
  };
}