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.