Size check

Apple Watch

The size is engraved on the back of the watch; the series is in Settings › General › About. 40/44mm: Series 4–6 and SE. 41/45mm: Series 7–9. 42/46mm: Series 10–11. 49mm: Ultra. Series 12 46mm needs its own case.

Galaxy Watch

Settings › About watch shows model and size. Watch 9 and 8 share cases in the same size; Watch 7, 6 and 5/4/FE each have their own; Classic and Ultra models are separate.

Phones

Pick the exact model: iPhone 17 Pro and 17 Pro Max, Galaxy Z Fold 8 and Fold 8 Ultra, Razr and Razr+ are all different bodies.