The right size comes down to how much you carry day-to-day. If you tend to carry a laptop, water bottle, and extras, a large tote gives you the room without the bulk of a backpack. A medium shoulder bag suits most daily essentials - purse, phone, keys, and a few extras - and is the most versatile size across smart and casual occasions. For evenings out or travel days where you're keeping things light, a small crossbody or clutch does the job neatly. When in doubt, go one size up - a slightly larger bag is far more useful than one that's just too small.