Obviously, dreams are highly personal so the first thing to do is think about what it mean to YOU specifically. For example, a black dog could be the comfort of a childhood pet or terrifying. Having said that, there are loose interpretations that I can suggest that you might relate to.
Bears often symbolise power or imbalance of power. Seeing a bear can indicate you or someone you know has too much power or not enough (again, it depends on the dreamer). Likewise, arms and hands can often represent effectiveness/productiveness or your ability to "hold on" and retain. The fact that your arm was smaller than normal may suggest that you feel your failing in one of these aspects or that you don't place much significance in them.
In terms of the man in the bear suit, there are lots of possibilities. For example, both men and bears can represent strength, aggression or protectiveness (or other associated traits). Try and think about the way he acted. Did he show any traits that really stood out?
I'm sorry I can't give any more details than that. As I said before dream interpretations are highly personal and given that I don't know you or your situation, these are only suggestions.
(Also, spirit guides take many shapes and forms. Some are animals, some are people. Each to their own. A guide is really a being that walks with you through life and facilitates your growth. It doesn't matter what it looks like)