Size depends on how often you want to clean the socks, how many spare socks you have, and how much load is going through them.
You don't want them clogged since it defeats the purpose. Some folks have many extras and are willing to swap them out quicker.
200 micron I think will clog pretty fast on a fully stocked tank, while 500-750 will take a lot longer. Me....I would want to change them every day, but every few hours!
http://www.monsterfishkeepers.com/f...40-A-Filter-Sock-Thread&highlight=filter+sock
Here's a great thread on socks. I bet you can find what you want here.