I got a Size 3 at first following Support recommendation but instantly regretted it, sizing is a bit tight, especially for large neck breeds (mine is a Bull Terrier - get size 4!).
I love this thing, I wish he could wear the black one but it's very hot here so I got the coyote one. I am considering getting a second one as a backup.. I can't tell how well it works because I can't wear them (lol) but my dog used to start getting upset when driving about 40kmh on the Scooter and now I tested this to 80 and he doesn't really anymore.
I kinda wish the Goggles had some kind of clip maybe that could hold the goggles to the front loop of the ear pro but i'm not sure if it's actually a good idea.