Is Vivosmart 4 too small?

It's not so small, I wear it as a man. But that's just my opinion.

Regarding the Pulse Ox, it's working for me, but when you want to use it for tracking pulse ox leves during sleep then be aware that the vivosmart4 is only capable to measure around a total of 4h during the sleep. so you will not receive a measurement for the complete night. 

I do not recommend it, since it does way too low values. If this is your only use case, I recommend getting a more sophisticated device from the medical field.

Pulse etc are fine, though. 

Thanks. 

4 hours are long enough. I am just curious how I sleep is. 

Currently I have a Vivoactive HR. It still fits my purposes but blood oxygen during sleep.

The 4 hours duration starts when it believes you fall into sleep or the time you set as bed time?

Thank you. I use it very casually. I am only curious how my blood oxygen during sleep like.

In GCM, you just setup your time when you're going to sleep. The Vivosmart4 then starts the pulsox measurement when hitting the sleep time. As long as you move your arm, Vivosmart 4 stops the measurement and continues it when you are not moving.

For tracking of spo2 and during sleep only, I think it is alright.

Do not buy the Vivosmart 4 primarily for sleep Ox tracking. In order to have reasonably accurate tracking while sleeping, you have to wear it snug, too tight to be comfortable (tight enough that it can cause some numbness/tingling in the morning - you don't want to be compressing nerves and obstructing blood flow in your wrist every night on an ongoing basis), else you'll get tracking drop-outs and suspect data even when it is functioning. Like sleep tracking in general (on all wrist-based trackers, because monitoring brain activity is the only way to accurately and reliably measure sleep stages), the sleep pulse Ox feature is more of a curiosity/gimmick than a truly informative feature. I used it for a while, but it really wasn't worth the additional battery drain.

The instantaneous pulse Ox feature is better - you can momentarily wear the watch snugly enough for a good reading, and while it's not quite as good as an actual medical device, it generally gives comparable results (I have a medical finger pulse Ox meter I've used for comparison, and the VS4 usually gives the same reading or 1% lower if I tighten it enough, though I'll note again that I have to tighten it to one notch more than I do when exercising to ensure a good heart rate reading, which is not comfortable and possibly not safe for extended periods of time; if I don't tighten it enough, it's off by as much as 5%).

There are two sizes, and the band circumference range for both is listed on the Garmin store page (the maximum wrist circumference for the large size is 215 mm).

thank you Johann7 for detailed informaion.

I don't think I can wear it too tight. Right now I have old model VA HR which I still like it because I don't do much on it. I loose the watch belt during sleep. So, yes for sure I cannot not wear it tight.

for the blood oxygen checking during sleep, like what you said, it is just my curiosity, not serious. Other than this curiosity, I have no need for a VS4.

thank you again.