Hey Thad! I'll answer this e-mail rather briefly because I've recently just purchased a great hat while thinking about these issues. I should add that wearing a hat that is too small gives me a headache.
I tend to favor hats that are a little loose at first and polyester. The material breathes better and lasts longer and will not become too tight over time. Also, you can use some of our poly specific techniques for shaping the dome. The hats will naturally fit to your head a bit over time, not as much as their wool cousins, but just enough to conform to your head if you wear the hat enough.
Best of luck and thanks for reading!