One hat size (cowboy), in 8th's , for instance 7 1/4 to 7 3/8, changes the diameter of the sweatband by 3/8 inch. Just a bit of trivia to say that going from 7 3/4 to 8 1/8 means stretching the sweatband 1 1/8 inches. It might could be done, but it won't be pretty. You will be able to see the outline of the sweatband on the outside of the hat. If you get in touch with your local retailer, they might can order what you need. As far as I remember, the major manufacturers don't deal direct. Try to talk to the larger retailers, they might have a better relationship with the manufacturers. They are probably going to be ordering their felts in the next little while, so you might have to wait a bit to get the hat in.
Oh, I worked for a western store here in Beaumont, Texas for almost 7 years while I was in high school and college. I've steamed many a hat.
Kirk
One other bit of Trivia, Ban Blocker, "Hoss Cartwright" of "Ponderosa" fame wore a size 8 hat.