Quote Originally Posted by Dan Fromm View Post
Well, the normal focal length for 20x24 inches is approximately 800 mm. The normal focal length for 20x24 cm is around 300 mm. A portrait lens would be as much as twice as long as normal.

There are very few 1600 mm lenses available, you might have to settle for a 1200, e.g., Apo-Ronar or Apo-Nikkor. There are more, but not many, choices at 600 mm.

To get a better idea of your choices, see, e.g., http://www.cameraeccentric.com/html/info/goerz_3.html . Rodenstock and Nikon process lenses were made in a similar range of focal lengths and have similar coverages.
Given that it's conceivable that a portrait taken on 20x24 might be close to life size, then the question becomes much simpler. Since the perspective is determined by the distance between the lens and the sitter, then half that distance will give you the focal length. If the distance between the lens and the sitter is 1.6m, then the bellows draw at 1:1 will be equal to that, and the focal length will be 800mm.