I've used name brand thundershirts on dogs before, so I'm familiar with the concept of how they work. I knew I needed it to fit snug, as that is what helps calm the dog, so for my girl I ordered the large size, which was listed as fitting a 41-64lb dog with a 25-30inch chest. My dog is 46lbs and her chest measures 28 inches so I thought this would be a good choice.
Now, my dog is a Weimaraner/Pitbull mix and she has a very deep chest but a slim waist, so I knew it might fit a bit weirdly, but I expected the chest to fit just fine, just the waist might be a little loose. The large size lists a boxer in the dogs it will fit, and they have a similar chest to waist ratio to her, so I figured it would be fine.
Out of the package, the instructions on the box weren't great, and the section that as it turns out is for if your dog wears diapers, wasn't mentioned on the box so I had to come back to the listing to even figure out what that extra spot of velcro was for.
Once I figured out how to get everything put on, I quickly realized that the vest is too big. You're meant to be able to adjust the vest by pulling the fabric tighter and velcroing it in place, but I had to pull the material tighter then the velcro would securely hold, to get the desirable level of snugness, and even then it wasn't as snug as the thundershirts I've owned in the past. I went back and double checked that I'd ordered the right size, because a large is listed as being able to fit a boxer, who have deep chests and slim waists.
Unfortunately, I don't think this will be very useful for my girl, because it just won't fit snugly enough to give the desired calming effect. The entire point is to feel like a hug, but this fits like most of her winter clothes, and the stiff velcro panels make it a bit bulky as well.
Currently, the price is basically the same as the name brand thundershirt, so I think in the future I would just get one of those.