The smallest skimmer I have tried that stunned me with its performance was the one found in the link.
http://www.northcoastmarines.com/pics/jst1001.jpg
It was the best skimmer I have ever tried and impressed with it performance, as long as the water level is constant. It comes in a very small foot print it is only 31/2 dia reaction chamber unlike what is mentioned in the website.
If I have a nano someday, I'll use these for sure.
Venki