Well, I get c. 5'-8" -- in his shoes. How about you ?
I think my method is sound: Transfer the known height of the chair, using the vanishing point established in the photo,
to the centerline of Wright's figure; apply the known measurement; using the same scale, determine the remaining height of the figure.
Of course, there are variables which could seriously diminish the hoped-for accuracy. For instance, if the figure is leaning toward the camera
it could easily add, say, a half inch or more Wright's height, by this method.
Mr Williams seems to be lifting the chair he's holding, off the floor. Are they discussing the furniture ?
SDR