I also shoot a SBE. I was told a Gobblin Thunder .655, ported would be the tightest to go. Any thighter would be worse than too big.
I shot several different makes and size of shells with it. ( Some sporting goods stores will sell you a mix of shells.) I got my best pattern with #6 HV federal prem. I've been told that #6 don't have enough knock down,( brush, long range, etc.) so I might go with # 5 also.
With my Benelli I can shoot untill I'm tired and not hurt, have some fun and shoot it up !!