Cirle hook size for live shrimp?

I have always used a #6 treble hook when fishing live shrimp. It is a pain to unhook some of those fish on the treble and unfortunately some of them end up dieing. I have been thinking of switching to a circle hook for live shrimp but I am not sure what size to use. What size circle hook would you suggest for live shrimp under a popping cork? Also are there any pro's/con to switching from a treble? Is there much difference in hook up ratio?