It's a Baskerville size 4. Her nose length is 2.5-2.75, and the circumference was 11.5. So it fits, and she CAN breathe and pant.... but, my concern is that she might need to pant more/bigger, if that makes sense. Because...
This is a dog known for her giant tongue and big ol' mouth when she pants:

So here is the muzzle on her:


See it enables panting, but not the exaggerated pant I'm used to seeing from her. What about on a hot summer day, would this be a problem, or is it fine as long as she can pant some:


And this is video of her actually panting in it (I can't figure out how to embed it, you have to click on the picture and it'll take you to the video). She IS panting, but I feel like her natural pant would allow her mouth to open much wider, and her little cheeks puffing make her look like she is straining. Is this normal/okay, am I overreacting?

The size 5 is definitely too big though; too long and way too much room for her to get her paws in and get it off. So if this size 4 is not the right fit for her, then I'd need to find a different muzzle altogether I suppose.