Apple Watch sizes

I recently purchased a new Series 3 42 mm with an aluminum case. I then noticed several Apple Watches that appeared to be larger than mine, and asked an owner of one of those watches if that was in fact an Apple Watch. He assured me it was, but I forgot to ask the size of the watch. Looking online at the Apple Store, I only see a 38 mm and a 42 mm. So, are there 3 sizes of Apple Watches? If so, how does one go about ordering the largest of the sizes?