Trying to get this darn thing sized.. well, all I have to say is measure well. It's entirely possible that the muzzle won't fit the dog's face so well, especially if he's a mix, like mine.
The shape of the muzzle is what makes it challenging; my dog (dane/rottie/shepherd mix) wouldn't fit properly in the 5 or 6. They're not really well suited towards a broader head shape, and there is no room or ability for the product to be adjusted at the bridge of the nose, which would be ideal.
If your dog has hanging jowls, like mine, they'll have to tuck up and shove underneath.
Also, considering what was supposed to be the proper nose length shoved right under his eyes, getting in the way of his line of vision.
Another area of concern was the amount of opportunity I felt it was possible to rip off the muzzle with the space underneath, even with all the other hooks and attachments. Again, it might also be because the size that was closest to fitting a broad head (6) caused more space below.
My dog resembles Houdini over Cujo, but I sense that this might not be the best muzzle if you have an extremely aggressive dog. I've handled a few dogs that I know could easily get out of this. On top of that, small fingers, even hands, could easily fit into the gaps around the muzzle-- at least for the larger muzzle types. This is a concern of mine, as the dog I purchased it for, after a fellow rescuer recommended it, has fear of people. If someone were to grab inside, they'd get their fingers bitten.. yet it would be me, and my dog, on the line for their stupidity.
Note that this muzzle isn't one that would work to prohibit barking at all, as it allows for a lot of jaw movement.
Overall, this doesn't suit my needs.
These reviews are the subjective opinions of ChickAdvisor members and not of ChickAdvisor Inc.