My software is using the following function to build the input field for in this case... input field name is Title:

<label for="Title"</label>
<?php ItemForm::title_input();?>

I do not know where it come from among the hundreds of files. All I want is to target this fields to set a maxlength. Is this possible?

Hmm well in plain html you can add a maxlength using the maxlength attribute. For example:

<input type="text" maxlength="128" ...>

But with the code you provided.. I don't know. You seem to be using a class named ItemForm? You should maybe take a look how that class generates <input> elements.