HCA: Hermit Crab Association

I was wondering if I was to get another crab what size it should be in comparison to my other crab(s). If they are about the same size I would think they would be less likely to hurt each other, but if I get one that is a significantly different size it would probably result in less competition for shells. Anyone have an ideas? I've heard of hermit crabs digging up other hermit crabs when molting and I wonder if that can be prevented if they were different or same sizes too.

I have a lil guy and a big guy that are best friends. I also have another bossy lil guy that likes to kick the bigger guys out of the crabbie cave lol. I don't think size makes to much difference as long as there are plenty of shells to chose from. I have noticed though that behavior wise, my bigger crabs hang out more and are more active and my smaller crabs tend to hide most of the time.