If you use Safari as your RSS reader of choice, and you prefer using the keyboard over the mouse, here’s a handy time-saver. As you’re aware, there’s an article length slider on the right hand side of the RSS display page. You might think you’re forced to use the mouse to change the slider’s value, but that’s not the case.
Assuming you’ve not changed your machine’s stock Full Keyboard Access settings (in the Keyboard & Mouse System Preferences panel), and that you haven’t enabled the “Press Tab to highlight each item on a webpage” option on the Advanced tab of Safari’s preferences, here’s how to reach the slider via the keyboard. After your RSS page loads, press Option-Tab until you see the slider’s blue ball highlighted. (If you just press Tab, you’ll only cycle through text boxes and lists.) To change the slider’s setting, use Control-Left Arrow (shorter articles) and Control-Right Arrow (longer articles).
If you have set Full Keyboard Access to All Controls, then you’ll only need to press Tab, but you’ll need to press it many more times, as you will literally be selecting anything that can be clicked on the Safari interface and loaded web page. Alternatively, you can enable the “Press tab to highlight each item on a webpage” option in Safari, and then you can just press Tab, not Option-Tab (and you won’t have to tab through nearly as many items as you do with the Full Keyboard Access solution).
And yes, you could just reach for the mouse and drag the slider, but not everyone likes to remove their hands from the keyboard for such a simple task.