At first it was a challenge to find the prefect size for my wrist but thanks to Ken for his suggestion. So if you are not sure of your size, email them first with your wrist size. Fits well on my wrist, comfortable and matches my watch. The only thing I'm concerned about is that since this is an elastic loop, if you take if off of your wrist, it stretches out and so at some point the elastic might get loose.
This is an interesting strap that I have installed on my Sinn 556. I must say that it changes the wear characteristic of the watch as it is now more top heavy. I have to use it for a longer period before I can render a final judgement but for right now, it feels quite comfortable, although top-heavy, as cited above.
The quality is very good and the band wears comfortably.
The sizing guide was super helpful, and the band ended up being the perfect size. I have 6 inch wrists and the 120mm size fit for reference. It's incredibly comfortable and well made.