How do I increase the character limit for the default "Fixed value" of a contextual filter? I'm trying to make a view of my new products using the Content: Nid contextual filter.

In this filter I opted to provide a default value, with a type of "Fixed value". On the "Fixed value" field, only 27 values are added, after which no more are accepted. 

Apart from the character limit for the text field, is there another reason  I can't add more values here? If not, how do I increase the character limit so I can add all the values I wish to?
Why don't you write a small code snippet and pass contextual filters using hook_views_pre_view(), that allows altering a view at the very beginning of processing a preview.

function MODULE_NAME_views_pre_view(&amp;$view, &amp;$display_id, &amp;$args) {
  if ($view-&gt;name == 'YOUR_VIEW_MACHINE_NAME' &amp;&amp; $display_id == 'page_1') {
    $nids= get_nids(); //write a custom function to get nids.
    $args[] = implode('+', $nids);
  }
}