Check the Q&A at railroadiana for Q744 and Q1432. Also see this link:

http://home.mindspring.com/~railroadimages/headlight.htm

The headlamp in the link shows more detail than the available advertizing artwork - in the Ham catalogs, for example. I would think that you could use any dimensions or aspect ratio you want, because there really won't be anything to compare it to. I would start with the reflector and build around that. The glass concave reflector mirrors were available up to 18" in diameter. BB