There are two factors for finding a comfortable fit: Size and shape.
Size
The difference in wear or placement means we measure for a mid-nineteenth century had differently than we do for a twentieth or twenty-first century hat. The modern hat is measured just above the eyebrow. (This is also where many of us measure for bonnets. We want to keep you on your toes.) For mid-nineteenth century, we measure higher, at the hair line. In this illustration, we can see the difference between where the two measurements would be.
These higher, hairline measurements are often smaller than those taken at the eyebrow. A hat worn at this point can be slightly smaller to slightly larger for comfort. So, add and subtract an inch to your hairline measure.
For example: I am 22.5″ around at my eyebrows and 21.5″ at my hairline. The vast land of the internet tells me that the average woman’s head measures 22.5″ to 22 5/8″ around at the modern measuring point. So, I am about average. I comfortably wear a mid-nineteenth century hat that is 19.5″ to 21.5″
Here is my head with the tape showing where to measure. This is where I wear most CW era hats. This is the circumference of my head. My measurement is 21 1/2″. (Note: this is a full inch smaller than the modern measurement take lower.)
General guidelines I use:
- Small = Less than 21″ at the hairline (crown less than 20″)
- Average = 21″-22.5″ at the hairline (crown 20-21.5″)
- Large = Greater than 22.5″ at the hairline (crown greater than 22″)
Shape
It is helpful to know whether your head is more round or more oval.When looking from above, some people have rounder heads while other have more oval heads. I have an average oval head. Very round hats don’t work for me without adding to the lining.
Hat blocks can be more round or more oval with the same circumference. To illustrate: Both of these shapes to the right can have a circumference of 22.5″. Yet, the same hat would fit each head differently.
If I Had My Own Blue Box: 
Is there a way to make a hat that is slightly too big fit more snugly?
Yes. Adding a lining will help make a more comfortable fit. A lining with gathers or pleats adds fullness, which will make a hat smaller.
Thank you