The thing that matters most with a triangle bikini is the tie strength and width at the back and neck, not the cup size. Everything else is adjustable if those two bits hold. Thin, flimsy ties that twist and dig in will ruin a good costume, and no amount of pretty print makes up for it.
If you've never owned one, start with a fixed triangle top on a halter and back tie, rather than anything fully adjustable at every point, because too many sliding rings and clips means you spend the first swim fiddling rather than floating. A simple set with a bit of lining in the cup gives you somewhere to start before you know what you actually want from a bikini, whether that's more coverage, a bolder shape, or a specific colour you'll reach for every summer. Once you've worn one for a season you'll know if you want the briefs higher at the hip or lower, and whether you prefer ties at the sides or a fuller back. In an ordinary wardrobe, a triangle bikini earns its place alongside things you already own rather than needing anything new bought around it. A plain linen shirt worn open over it on the walk to the pool, denim shorts for the car park and the shop afterwards, a pair of flat sandals that don't mind getting wet, a soft cotton dress thrown over the top for lunch without getting changed. None of this needs matching or planning, it just needs the bikini itself to be comfortable enough that you're not adjusting it every ten minutes, which is really the whole point of owning one. Before you buy, check the width of the back strap sits flat rather than rolling, check the lining isn't so thick it takes an hour to dry on a towel, and look at where the triangle sits once it's on a body rather than on the model shot, because cup shapes vary more than brands let on. Press the ties gently too, cheap toggles crack in a single season of sun cream and chlorine, and you'll be back buying a replacement before the holiday's even over, which I've done more than once, annoyingly.
This suits anyone who wants sun on as much skin as possible and doesn't mind a bit of adjusting now and then to get the fit exactly right for the day.