Here is a comparison I recently created for my studio lighting class. It shows the difference in nine separate lens focal lengths, ranging from 20mm to 200mm, used for a studio portrait. The exposure and camera height remained constant, with the camera moving backward for each subsequently longer focal length to maintain the same eye placement in the image.
The difference definitely becomes more subtle above 100mm, with the main difference being the relationship between head and body size. My preference is in the 100-120mm range, which keeps the body size from appearing “too sturdy”, but I will routinely back up and shoot closer to 200mm when needed.
Canon 5D Mark III – 1/125 second, f/9, ISO 100