this is my way,
1. Use the Shear Tool and Perspective Tool in GIMP (or Photoshop) to adjust (projection) the Perspective view to Elevation view.
2. Export the modified image into a Rotoscope in the Model window in A: M and then create the model as normal.
3. You know the base dimension, scale Rotoscope to get the correct dimension.
4. Estimate the depth and height of model by adjusting the proportion in choreography.
(please excuse me for grammar, I wrote in Thai and then "google translate")