A fuly grown Fischer grows to the size of a small fresh-water otter. They are incredibly smart, cunning, fearless, and powerful.
They make ready use of their sharp teeth and claws, but it's greatest strength is it's smarts. They are decimating an unchecked porcupine populations in the north simply because the porcupine has had no real natural enemies until the Fischers showed up: they flip the porky on it's back and attack the underbelly.
They could easily kill and eat a house cat.