How do I increase the character limit for the default "Fixed value" of a contextual filter?
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);
  }
}