It seems this value is not accessible, neither on the client, nor on the server. Mendix hard codes this into the pages of the application: if you deploy your application and take a look at /deployment/web/pages/LANGUAGE/MODULE/PageName.xml, you will see something like:
<div data-mendix-id='27_6' class='form-group mx-name-textBox6'> <label class='control-label col-sm-4'>Naam</label> <div data-mendix-id='27_17' data-mendix-type='mxui.widget.TextInput' data-mendix-props='"insideFormGroup":true,"attributePath":"Beheer.Contactgegeven/Naam","placeholder":"","maxLength":200' class='col-sm-8' tabindex='0'> </div> </div>
I'm afraid you will have to come up with a workaround. The easiest one would be to hardcode the max length to 200 (which is the default setting in Mendix).
Furthermore, this seems like a perfectly valid use case, so you should also submit having access to this data as an idea on this forum.