check out "filter bags" on mcmaster.com for a pretty decent selection.
In years past I tried everything down to 1 micron. I found the 1 micron clogged easily within 1 day and that I had similar problems up until I got to the >25 micron range. In the end, I found that 50 micron was the smallest I could get away with using without having to clean it more than once a week.
The smallest known bacteria is in the .4 to .5 micron range. I believe the nannoscloropsis (however you spell it) is one of the smaller phytoplanktons (and is a major strain used in nearly every plankton supplement on the market) and it is about 4-5 microns in size range. Many zoolplanktons and rotifers are larger than 100 microns. It would be safe to say that all copepods that you can see with the naked eye are larger than 100 microns. People are concerned about their refugiums ability to add diversity to their aquariums microfauna, however if you use a filter sock, you are probably going to severly limit what remains in your system. With that in mind, I stopped using a filter sock at all.