Instruction: Generate an answer to the question by utilizing the information contained in the document.

Input:
Question: I don't see a sizing guide. How do I know to order Regular or Large?
Document: It's also great for working on my laptop - keeps me from sitting in uncomfortable positions.The sizing can be a little confusing (and isn't really well-described anywhere...). If you have a smaller neck, regular is perfect. A large is a must have for men, or anyone without a thin neck.
I am a woman with a long slender  13 " neck. The regular collar barely fits around and is so short & narrow it gives virtually no support. The product description measurements are the same for both the regular and large and are inaccurate. Must be the size of the box. I don't know if the large is taller as well as longer;  the regular might work on a very small petite person or child. Does not even come close to the  actual photo on the box.
The idea, the design of the Caldera is excellent but:Not until I received the Caldera Neck rest I discovered that it was a too small size. There was no way to know what size I was ordering online. And now I am disappointed and wish I could replace it with Size Large. regular is a way too small for any neck, I believe except perhaps for a child of 12 years old. I do not understand why Amazon keeps the sizes in secret from buyers.
Ordered the Caldera Releaf Neck Rest. Measurement showed to order a Regular - way too small. Reordered a Large and works well. With the correct size - product is what I expected. I always look at Amazon first before I place orders and usually place the final order at Amazon. Love their site and service and opportunity for free shipping on most items.
If you have a long neck this product isn't long enough to give any support. My head still flops around. Probably would be ok for most people but it's hard to know before you order it.
I fly across the continent regularly for business. Like most frequent fliers, I consider three gadgets helpful. They are, in descending order of importance, noise-canceling earphones (likePanasonic RP-HC55-S Noise-Cancelling Earbud Headphones (Silver)), a neck pillow and an eye mask.I used to use a regular neck pillow. It was quite comfortable but way too big to fit into my computer bag.
I have a regular that I have used for 3-5 years. It prevents motion sickness and migraines in cars and planes. I won't travel without it. It has been to Australia 4 times in the last year alone. Ordered a second collar in the same size. Sizing is all wrong. 1/2 inch too tall in the centre and 1 1/4 inches too tall under the ears. Catches on my collar bones, flares out towards my shoulders and holds my head in an uncomfortable position.
My husband and I both have ther Releaf Neck Rest. We store them in our carry-on luggage so we don't forget them. The Neck Rest is comfortable and very effective. No head bobbing around. Makes it easier to get comfortable. Other review said to order big which was good advice. Medium for me and large for my husband. We are average build.
The neck rest is better made, looks good and is less obvious to wear than the white ones available at a drugstore or from your doctor. Be sure to order the large one if you're a man.It does insulate, so after while it does get warm and I tend to sweat a lot but it washes well and holds its shape. I use it day and night if my neck hurts.I have ordered 3 so I have a fresh one at all times. I highly recommend this neck rest.
In examining the Caldera it looks like one end is chopped off- the end with the velcro strap. I compared it to a professionally fitted collar that I purchased from a medical supply store in a medium size. The padded portion of THAT one meets in the back and the velcro OVERLAPS the opposite padded end completely making a comfortable closure. The ad doesn't explain sizing but the photos show the brace completely encircling the neck with no large gap in back.
Answer: The sizing guide is difficult to find, but it is there. On the left of the product image are some small icons of the different product views. If you click on the one that shows the back of the box , you will note that Regular fits 12-15" and the Large 15 1/4 to 17 1/4 necks. Hope this helps.