Hi are you sure the heart rate monitor that tends to dropout is due to the apple watch and not your heart? I’m asking because my heart rate also drops from 123bpm to 60bpm while running but I also got this same heart rate dropout with my other 3rd party heart rate device.I am a runner (marathoner) and run with Apple Watch Series 3. GPS accuracy is great - I've run with Garmin and TomTom watches in the past. Only issue I have is heartrate monitor does tend to dropout, especially in colder weather when you sweat less. The Series 4 has a 2nd gen HR monitor, which I am hoping will fix this. Can't comment on size as I have small wrists and have a 38mm.