This is a quick and rough version of measuring for my Civil War Era hats. I have this really cute version of understanding how CW era hats sit and how to measure for mine. But, that is going to take a couple days of computer work. So, for now, here is this version with quick, hair roughly tied up, taken in the bathroom photos of me.
Step one: Push what you know about modern hat sizes and fit out of your mind. Nineteenth century hats are not worn how modern hats are worn.
Measuring
Measure your head at the hairline. This is close to where most of your hats for this era will be worn. Measure here, parallel to the ground, or at least fairly parallel to the ground.
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.)
When selecting a CW era fashionable hat, subtract an inch from your head measurement. A 19 1/2″ to 20″ low brim or Mousquetaire hat fits me.
Another thing to know about your head is whether it is more round or more oval. This is because some of my blocks are more oval or more round. If you know this, I can advise you if this is a factor. Here is the top of my head. I have an average oval head. Very round hats dont work for me without adding to the lining. 
Fit
The majority of Civil War Era fashionable hats are worn high on the head. This is roughly at the hairline for most people. There are some that are worn higher and some that are worn lowered. Here is a sampling of photos from my “ooo, I need to make that folder” in my tablet. Notice how high each sits and that some of them sit tilted back (admittedly this may be for photographing purposes) which may be earlier.
Here is an assortment of illustrations to compare:
Here is a look at how I would place a variety of styles on my head. Please note, this evening I do not have a hat that is my size on hand. I just want to get this posted due to the number of questions and comments regarding fit and measuring.
This is a low brim hat. It sits at my hairline. This one is an inch too large for me.
This is the Mousquetaire style with the higher crown and narrower brim. This sits at my hairline as well. Illustrations and photos suggest this style sits at or below the hairline in the era.
This is the brimless toque style. Photos suggest this should sit lower on the head than other fashionable styles. For this style, select a hat the same as the measure of your head or a little larger if you want to bind the crown edge. (so tempting.)
If I Had My Own Blue Box: 
Thank you. This helps a lot.
Question does the hat come lined?
Elaine Hale
Sent from my iPad
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The hats and bonnets do not come lined unless I’ve decorated them. Each is a straw ready for you to finish to suit your impression. I am happy to offer recommendations as you proceed.
Recommendations would be very helpful.
RESERVED for LJS. Civil War Era Straw Hat – is this a name of the hat or is it reserved for certain people (LJS). (LJS) meaning? I really like this hat but is it appropriate for an older women.
Any Etsy listing marked Reserved is for someone specific. I have a list of pieces from last year I am working through.
Thank you, now I Understand.