You cannot place more than 255 characters in the text field of list with
content type as Document.

support.microsoft.com/default.aspx

Other Lists support up to 5000 characters I understand.

Well I would like to use a text field with more than 255 characters.
Does this mean that I cannot use the Page Layout from the publishing
feature (which inherits from document) nor use SharePoint designer to
add the text column placeholder?

Is there a way to make a Content Type that descends from "Item" that I
can use within sharepoint designer?