The Cooper's Hawk looks very similar to the Sharp-shinned Hawk.
Their size is not always indicative because a female Sharp-shinned Hawk can be as large as a male Cooper's Hawk.
However, the Cooper's Hawk's tail is usually rounded at the tip (square with Sharp-shinned Hawk) and the middle tail feathers are usually the longest (Sharp-shinned Hawk's outer tail feathers are usually the longest) .
In addition, the legs of the Cooper's Hawk are thicker and usually Cooper's Hawk has a more "fierce" facial expression.
The Ramble, Central Park, Manhattan, New York City, New York, USA, 28 February 2007