My wrist is 5.5" and I have a (tighter fitting) size S in the demi pave. Mine was purchased in Japan so it's actually marked "15" as in 15cm.
It slides up and down my wrist an by an inch or two but there's no noticeable gap and I would say it's a snug fit.
You can visually tell how small a bracelet is by how many segments the head and tail overlap by - mine overlaps by 4-5 while I've seen some others overlap by 2-3. This accounts for the size variation within sizes.
I preferred the tighter look and fit knowing it would eventually stretch out. If you prefer more overlap on the head and tail, or any movement at all, I'd push you toward getting a tight M.
I think if you were to go with S you'd end up stretching out the bracelet sooner. Ofc, if you like the stretched out look or the feel of no movement on the bracelet you can go with S.
Hope this makes sense!