One of the great things about this forum is the wealth of knowledge from my fellows:  Tony Thompson's advice on metallurgy; George Toman's note concerning pliers; Bill Welch's use of larger diameter wire; Bill Pardie's suggestion to file rather than pinch the handle; Dennis Storzek's 1922 ARA drawing of door handle dimensions.  I may incorporate all of these ideas into my next attempt at door handles.

And, as Bill Pardie suggested, in the case of a molded plastic model like the BLI NYC steel box car, the handle size was, in fact, determined by the molded door anchors.

Thanks for all the advice!
Michael Gross
On The Road - Lake Tahoe, NV