My app is using 25k docs. What I described in the article is very basic, the app that you can try at http://plixitank.heroku.com keeps a window of the 25k most recent items from Plixi and erases older stuff. That's enough for several hours' worth of search history.
Trendistic is a special-purpose app with several millions of documents. However, the contest accounts are limited to 1M documents so don't worry much about the size. You could try an interesting approach at indexing tweets and should be more than fine choosing up to 1M tweets with some criteria, be it recentness, popularity of the author or something else. You can contact us directly if you have other specific questions, support [at] indextank or through the chat box on our site.
