Which version with small wrist (~16 cm)?

If the bigger watch ends up bouncing more, then heartrate measurements may be affected

Check size in your watch description, check the same for watch you want to buy and you will know.

My wrist is about 20cm, maybe a bit smaller. I decided to get the 51mm version because of screen size. I use it primarily for hiking and the readability of maps is crucial to me.

Heart rate measurement is accurate if the wristband fits tightly. But that is not a problem with the synthetic band.

My wrist is 17 cm and the Mk3i is perfect. Use it as every day watch for all occasions. Love it! 
Had Mk2i and Mk1 before and think the size is good…

Here's what the 43mm (top) looks on my wrist which is about 15.75 cm. Bottom is my partner's 51mm on me - hope this helps. 

Thank you for the comparison! I'll stick to the 43 mm version. It makes no sense to wear such a big watch with my small wrist. I guess, as a man I didn't exactly win the genetics prize when it comes to thick wrists. I've been doing deadlifts and grip strength exercises for years, but the wrist just doesn't get any thicker (close to the hand of course). 

I just wish there was an inbuilt flashlight in the smaller version. I mean, the smallest Fenix 7 and Epix 2 Pro versions have one as well and the Descent Mk3i launched way later than these 2 watches...

Sure thing - glad to help. The flashlight on the 51mm is pretty cool. Admittedly, my partner thought the 51mm was too big for his wrist (about 18cm, obviously bigget than yours), but he loves it and wears it every day. My issue with the size would have been the weight as it's a bit chunky for me to wear every day, but if you're used to a larger watch, it may to be an issue for you. At least they have a black one int he 43mm, if you wanted that option, even though, as you said, no flashlight.

I see! Yeah, my wrist is about the size of yours. You simply cannot force what is not possible. That's the way it is, I have to admit. I mean, it's no good if the watch keeps slipping when I'm doing any kind of sports and the sensor doesn't achieve the adequate measurement results that it would if it was always resting properly on my wrist. Personally, the weight would bother me less.

If there were a 47 mm version, that would certainly still be feasible, perhaps in the next iteration of the Descent.

I'm currently thinking of getting a Garmin Epix 2 Pro Sapphire in 47 mm (i.e. in the intermediate size, which is not available with the Descent and still get the flashlight in this way) and a used Descent Mk1 or Mk2 on top of that. Then I would end up with almost the same price - of course, I wouldn't have a Mk3, but the Epix 2 Pro Sapphire is in no way inferior to the Descent Mk3 in terms of everything that happens above water.

I want to upgrade now. The Fenix 5s has reached its age. ^^

Just FYI, in case you don't know, that the MK2 is not touchscreen - this may not bother you, but it would drive me nuts after having only touchscreen Garmins, including the MK3i. If you care about air integration, you'd be losing some options with the MK2 T1 transmitter, but you can get some great deals on used versions right now, so I can understand that being an option. Good luck making a decision. 

Wrist 19cm, Descent Mk3i 51mm.