I’m trying to decide between a 4/0 and a 5/0 range roller XL jig head for 5.5 spunk shads. What size hook do you think is best?

I'd go smaller personally.  You want as much the tail free to waggle as possible so the shorter hook will be better.  

So 4/0? What about for a 4.5 spunk shad? 3/0 or 2/0? 

I would want at least a 4/0 gap for a bait that thick.  You just don't want a longer shaft length, that could stiffen the bait up.