You need to add a "config" parameter to your field object:
{"field":"article","caption":"Article", "inputTVtype":"listbox","inputOptionValues":"@EVAL return '-- select resource --||' . $modx->runSnippet('select_article');","config":{"typeAhead":true,"typeAheadDelay":250,"minChars":3}}
The "typeAheadDelay" and "minChars" options do have defaults already set in modx but can be overridden with the above config.
Additional information on Ext.form.ComboBox:
http://docs.sencha.com/extjs/3.4.0/#!/api/Ext.form.ComboBox