Went browsing at a JLC boutique for a Reverso and here are some wrist shots to compare the sizes. My right wrist is 19cm, though probably rounder in shape rather than flatter.
Hope this help anyone in the meantime .
From top to bottom on wrist photos:
Reverso Classic Medium Thin Q2548520
- Dimensions (L x W) : 40.1 X 24.4mm, Thickness : 7.4mm
Reverso Classic Medium Small Seconds Q2438520
- Dimensions (L x W) : 42.9 X 25.5mm, Thickness : 7.5mm
Reverso Classic Large Q3828420
- Dimensions (L x W) : 45.6 X 27.4mm, Thickness: not sure
Photo off the wrists in no particular order