My content type has a taxonomy auto-complete field. My problem is that some content requires like 10-20 taxonomy terms where my input field for it is way to small.
I don't see in my 'manage fields' settings where I can define this length.
You can use the Entity Reference module; it allows the size of the autocomplete input field to be changed.
Just saw in a similar question this info : https://www.drupal.org/project/autocomplete_deluxe "This is an enhanced autocomplete element, that uses the JQuery UI autocomplete. It will also implement a widget for taxonomy."