We use a closed SharePoint blog for a client with NT security and use the RSS to display the posts on a remote .NET portal. The RSS by default shows items only for a week and uses only 25 items, these setting couldnt be changed so we thought.
but after some extensive SharePoint clicking I finally found the settings page. It wasn’t at the RSS part of the site settings page of the site, it was at the site settings of the List of blog posts. So go to your list, list settings. under Communications you’ll find RSS settings. Here you can edit the “Maximum items to include”, “Maximum days to include” and which Columns to include.