According to the stories, as a wolf, he is huge. The size of a horse or even bigger. The shamans believe this is actually his true, natural form. Meanwhile stories record that, as man and werewolf, he is merely big, almost 7 feet tall.
Fenrir is a warrior god who values strength and toughness: to appease him, enemies should be defeated and made to serve the werewolves, as that's the natural order of things. The strong rule the weak. However, he also has a bit of a mischievous streak in his character and likes to tease both his followers and his enemies, sometimes in a benign way, sometimes a bit more edgy. As such, he has a good sense of humor and can take the occasional joke. He has little interest in weaklings, but finds it amusing if they try to be tough. He's not very vengeful and seldom holds a grudge.
He's not a complicated god. Although he enjoys a nice ceremony or a sacrifice, he has no need for overly complicated rituals. Which is logical, as he is more inclined to be god on the side of chaos than on the side of order. Too many societal or religious institutions make him wary.