Any CMS fields that were populated with text prior to the character restriction will remain the same. You will need to go in an shorten post summaries to 150 characters. If you were try to save a CMS item with your new character restriction in place, it would not allow you – as it’s longer than 150 characters. See here:
If your descriptions are shorter, I don’t imagine you will need the ellipses (three dots). However, if you really want that effect, I understand Webflow recently added a new text feature pertaining to text wrapping/overflow – but I’m not aware if it handles ellipses – so here’s a working solution: