In the Garmin store I have an app that has the following app setting:
<settings>
<setting propertyKey="@Properties.LC" title="@Strings.LC">
<settingConfig type="alphaNumeric" maxLength="100000" />
</setting>
</settings>
The maxLength attribute works on Garmin Express Windows, Garmin Connect Mobile Android and Garmin Connect Mobile iOS but not on Garmin Express Mac. On Garmin Express Mac (version 7.4.1.0) the maximum string length is always 256 characters no matter what maxLength attribute is set to.