what to put in a refugium?

ide toss in atleast 4 inches of sand, and soem mangroves and cheato if you have the room for it. If not just toss in Mangroves.

 

I don't think a refugium will make a very good filter. Filters need high flow and refugiums need slower moving water. I would skip the idea of filtering anything in it and add some sand/miracle mud, rubble rock, macro algae(chaeto, caulerpa, gracilaria, etc...), some mangroves(ebay has very cheap mangroves), and a small light. Don't put any predators in there that will eat pods and everything benificial you're trying to culture in there. You could buy some copepods or mysid shrimp and stock it, but even easier is to use some established sand from your tank or someone elses and some macro algae that is already flourishing with pods. I sock mine with extra pods by sucking them up with a turkey baster or using a net at night while viewing my tank with a flashlight.

What are the dimensions of the refugium?

Adam