I think it could be a good idea to contact Lyman directly. Some folks have reported ordering the particular size that they want for a very modest price. I'd figure out what basic expanding diameter I'd want and then let Lyman pick the diameter of the larger step. Could maybe let them know how deep the bullet will be seated, unless someone wanted it deep in the case past the neck. Sometimes the cast bullet is larger than the bore diameter, for good reason, and the expander could be sized for the bullet and not just generically for the stamped caliber.