refugium size ?

Instead of starting a new thread I figured I'll just bump this one and add to it. Without research I trusted my LFS to provide me with what I needed to get my system up and running. At that time he recommended a 10 gallon HOB fuge. I've had to DIY the thing to death and move it to another room off of the sump. It grows chaeto and critters wonderfully but I doubt it's doing much for the DT. I want to DIY a new fuge and am wondering what size, fuge only, is everyone running for their system. How much macro algae does one need to grow where it becomes a viable part of your filtration system.