I haven't seen the number of items in a feed make much difference. I set it high on the feeds that get a lot of traffic (keeps people from missing items) and low on the ones that don't. Eleven isn't pretty small, so don't worry about it.

Look at the feeds from major news or technology blogs. They have hundreds of items sometimes. You might want to limit the size of the feed if it became too large for your visitors to download though.

The other thing you want to keep in mind is that your feed is a service to your users that lets them consume your content in another way. It gives them a heads-up when something new happens on your site. It's a reflection of the activity on your site. Don't artificially inflate the feed.