You can use a 20mm width band with:
* Galaxy Watch4 Classic (42mm & 46 mm)
* Galaxy Watch4 (40mm & 44mm)
* Galaxy Watch Active 2 (40mm & 44mm)
* Galaxy Watch Active (40mm)
* Galaxy Watch Sport
* Galaxy Watch (42mm)
* Galaxy Watch3 (41mm)
You can use a 22mm width band with:
* Galaxy Watch3 (45mm)
* Galaxy Watch (46mm)
* Gear S3 Frontier
* Gear S3 Classic
Source: Reddit