I measured in advance the things I would need to carry in this bag. I chose it in part because the advertised measurements would work for me. Snugly, but work. Well, unfortunately, the advertised width is not correct. It's listed as being 5.1 cm, or about 2-inches. Two inches would have worked for me but the actual width is about 4.1 cm max, or 1 5/8 inches. Not a big difference but enough so that my wallet and a crunched-up spare tube wouldn't fit. And big enough difference for anyone else to take into consideration if they're thinking of buying. The build of the bag seems just fine so I'm just docking one star. It's the width that's the problem. The height and length measurements are fine.
SIZE: Just right. I can fit 3 CO2 canisters, a tire lever, the CO2 head, and, most importantly a 25mm 700c tube. It's a snug fit, but, all these things fit. And, I can cinch it up very tight under the saddle using the 3 velcro straps. My prior bag (Topeak Aero Wedge Pack - Strap MediumO which BTD recommended, was big enough for everything, but, was too big and too wide and ended up rubbing a hole in a pair of my bib tights (inner thigh).
This is my 4th or 5th or maybe 6th saddle bag. Is it waterproof? Nope. Your stuff will get wet. But, I had "waterproof" bags in the past. They aren't. The zipper on the "waterproof" bag was incredibly finicky and I almost always had to take the bag off just to open the zipper, at least until the zipper failed entirely after only one season of very light use and relatively minimal wet days (here in CA). So, I'm trading waterproof for what appears to be a much more durable zipper.
FIT: 3 velcro straps which you'll likely have to cut (they're extra long to allow for fit) after install. I can cinch this bag up very high and tight under my saddle. It's out of the way, and doesn't;t rub against my legs.
Great appearance and it's made very well, but it's odd shape limits the stuff that will fit inside and it's a bit $$. I carry a multi-tool a small can of pepper spray, and my house and car keys. Since I went tubeless I don't use a seat bag anymore and I needed something. It doesn't attach to the head tube and will spin around and end up hanging on the bottom of the top tube unless you attach it. I used two strips of Velcro between the bottom of the bag and top tube and it did the trick, A 55 for the design and construction and a 35 for the lack of attachment to keeping it from spinning on the top tube.