If you live in a busy city, a bag like Marc by Marc Jacobs Classic Q Natasha Crossbody is going to be a staple for you. If you need a bag that stays close to your body while you run around doing all your important errands, then a bag like this would be the necessity because you do not have to mind so much compared to a clutch or a big shoulder bag.
This bag is the “perfect medium size” in the sense that you would not be tempted to stash everything inside thus rendering it heavier than its intended use. It’s not too small to where you can only carry a few things.
Something good always comes out of Marc by Marc Jacobs.