If you have to change dinamically the number of items that are displayed on your feed page or do you need to set their offset via Drupal 6 Feed Rss Views Module, than this post could helps.
After googling for a couple of hours without find any solution to this issue, i tried to solve it by myself.
I would like to pass custom number of items and their offset as arguments of an url.
Something like this
http://www.yourdrupalwebsite.com/rss.xml?items-limit=5&items-offset=5
This page is a custom feed page created by the drupal's admin side .
/admin/build/views/edit/(rss-feed) (in my case)
After that you'll have to use this little hack that change a little piece of code inside the file
modules/views/plugins/views_plugin_display.inc
I found this piece of code on the line 1861. So i changed the following default settings:
$this->view->set_pager_element($this->get_option('pager_element'));$this->view->set_items_per_page($this->get_option('items_per_page'));
$this->view->set_offset($this->get_option('offset'));
and i replaced them with:
$this->view->set_items_per_page(($_GET['items-limit'] > 0) ? $_GET['items-limit'] : $this->get_option('items_per_page'));
$this->view->set_offset(($_GET['items-offset'] > 0) ? $_GET['items-offset'] :$this->get_option('offset'));
Save the file and you'll be ready to call your feed page with your custom arguments with it.
http://www.yourdrupalwebsite.com/rss.xml?items-limit=5&items-offset=5
and check the response.
Note:
You need to filter and sanitize your $_GET input values for security reasons.
Enjoy