The one thing worth knowing before you buy a small bag is that it has to earn its size by what it actually holds, not by how it looks on the shelf. Before anything else, check it fits your phone with the case on. That single test rules out more bags than any strap length or colour ever will.
Getting the fit right is less about your body and more about what you're carrying, but there's still a body element to think about. A crossbody strap needs to sit at a length that lets the bag rest at your hip without the chain or leather cutting into your shoulder when you've got a coat on, so try it on over whatever you actually wear in winter, not just a thin top. Short handheld straps look lovely but mean you've got no free hands, which matters more than people admit when you're also holding a coffee, a child's hand or a set of keys. Watch for magnetic closures that pop open on a crowded train, flap fastenings that take two hands to manage, and anything with a single internal compartment, because you will end up digging for your card at the till while a queue forms behind you. A bag with even one small zip pocket inside saves you from that most days. If you wear glasses, check the bag can actually hold the case without the zip straining, that's caught me out more than once.
Where it struggles is against anything else fighting for attention. A small bag with hardware, chains, studs, a big logo plate, sits oddly next to statement earrings or a heavily patterned coat, because your eye doesn't know where to land. Plain and structured works harder with print, texture or jewellery than anything with its own decoration. It also disappears under a long, full coat, so in the coldest months you end up either wearing it over the coat, which looks a bit odd, or switching to something with a longer strap for that stretch of the year. And there are days it simply isn't the right tool. Anything involving a laptop, a packed lunch, gym kit, or a toddler's spare clothes needs a proper bag, not a small one asked to do a job it was never built for. I've tried forcing a week's shop into one and ended up carrying bread under my arm, which solved nothing.
It suits anyone whose day genuinely only needs a phone, cards, keys and a lipstick, and who's happy to plan the rest of the outfit around it rather than the other way round.