That is a hard question to answer because it really has to do with your bioload. It has to do with the number of fish in the tank, the amount and type of food you are feeding. If your load really high, then you go with a larger reactor but if your load is what would be sometimes considered normal, then a smaller reactor will work better.
In your system, I would say you will be using about 1000-1200 ml of biopellets. My suggestions would be the following:
External: http://www.coralvue.com/reef-octopus-br110-biopellet-reactor
In Sump... http://www.coralvue.com/biochurn-150int-biopellet-reactor