The size that matters most
Watch size is reported as case diameter (the width across the case from 3 to 9 o'clock, excluding the crown), but the spec that actually determines fit on your wrist is lug-to-lug: the total length from the top lug tip to the bottom lug tip. Two watches with the same diameter can wear completely differently if their lug-to-lug differs by 4-5mm. As a rule, lug-to-lug should not exceed your wrist width measured top-to-bottom (where the watch sits, between the wrist bones).
Modern watch sizes drifted upward through the 2000s; 44-46mm became normal, 38mm felt small. The 2020s have reversed this: 36-40mm is now the enthusiast sweet spot, 38-39mm is the most-recommended size for first-time buyers, and 42mm+ feels chunky outside dive-watch contexts. The Tudor Black Bay 58 at 39mm and Rolex Datejust at 36mm/41mm are the canonical first-watch sizes.
How to actually decide
Measure your wrist circumference with a tape (or a string + ruler). Then measure top-to-bottom where the watch will sit (usually 5-8mm narrower than circumference suggests). Aim for a watch whose lug-to-lug ≤ that top-to-bottom measurement. If lug-to-lug exceeds your wrist top-to-bottom, the lugs will overhang and the watch will rock on the wrist. Diameter is secondary: a 42mm watch with short 47mm lugs wears smaller than a 38mm watch with 50mm lugs.
Try watches on before buying when possible. Most authorised dealers will let you handle multiple references. The watch that feels right on the wrist is more important than the marketing image of how it should look. Thickness is a third axis: anything over 13mm starts catching cuffs and feels chunky; sub-11mm reads as elegant and dressy. See wiki: lug-to-lug.