Wrong! Anything under 50mm can have significant distortion (regardless of sensor size), and if used for portraiture... should be far enough away to minimize the distortion. Even a 50mm will do it if too close.
Focal length has nothing to do with this.
You can transform a picture taken by a 35mm, 28mm, 24mm etc into a picture of a 50mm or any longer focal length by just
cropping.
The more you crop, the more loss of resolution - however, assuming a perfect lens, there is no distortions introduced or removed. At all !
Even a 14mm (or less, but 14mm is kind of the limit of what still makes sense) will still have perfectly straight lines except for lens errors or if its a fisheye.
The issue is PERSPECTIVE, not lens distortion.
To make a portrait, you want to keep a distance of about 5 meters from the subject. A bit less might still work, and a lot more still causes little problems (people will look somewhat flat though). This limits the range of good focal lengths for portrait to 50mm to 135mm, for full body to face only, and all relative to full frame.
The famous 85mm f/1.2 Canon portrait lens, for example, is right in the middle of this.
Nikon also has famous portrait lenses, the AF 105 or 135mm f/2 DC lenses, which are thus a bit more specialized for face portraiture.
