Measure the OD To measure the outside diameter of a bearing place the jaws of the caliper around the bearing and close it until its a good fit but not tight, now read the value from the caliper.
What size is a 6203 bearing?
d 17 mm Bore diameter
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D 40 mm Outside diameter
B 12 mm Width
d1 ≈ 24.5 mm Shoulder diameter
D2 ≈ 34.98 mm Recess diameter